Fishkeeping

Fishkeeping is the maintenance of a small, closed aquatic ecosystem. The fish are the visible part; the actual work is keeping the water chemistry stable, and almost everything that goes wrong is a water problem presenting as a fish problem.

The hobby divides into branches that share very little beyond a glass box. Community freshwater — a mixed tank of small tropical fish — is where most people start and is the cheapest and most forgiving. Coldwater needs no heater and is generally larger than beginners expect, because goldfish grow. Planted and aquascaped tanks treat the plants as the point and the fish as accents. Biotope tanks recreate a specific stretch of a specific river. Marine adds substantially more expense, equipment and precision, and reef adds coral, which is a considerably higher bar again.

What people underestimate is that this is animal keeping. A tank is a commitment measured in years — many community fish live five to ten, and some considerably longer — with a weekly routine that does not pause because you are busy.

HobbyHQ covers fishkeeping at the level of the hobby: what it involves, what it costs, what the branches are, and the handful of concepts that determine whether a beginner succeeds. Species-by-species care, water chemistry in depth, disease diagnosis and equipment specification are a specialist body of knowledge, and this page is deliberately an orientation rather than a substitute for one.

1. 1

   Step 1: Buy a bigger tank than you were planning to

   The counterintuitive rule of the hobby: larger tanks are easier. A bigger volume dilutes mistakes, holds temperature steadily and swings in chemistry more slowly. A 100-litre tank is genuinely more forgiving than a 30-litre one, and costs perhaps 30 per cent more to set up.

   You will know it worked when: Small errors stop being fatal.

2. 2

   Step 2: Buy a liquid test kit, not strips

   You cannot manage what you cannot see, and ammonia, nitrite and nitrate are all invisible. Liquid reagent kits are considerably more accurate than paper strips and cost around £25 for hundreds of tests. This is not optional equipment.

   You will know it worked when: You know what is happening in the water rather than guessing from the fish.

3. 3

   Step 3: Cycle the tank before buying any fish

   Three to six weeks with no animals in it. This is the step everybody wants to skip and the one that decides the outcome.

   You will know it worked when: A working biological filter.

4. 4

   Step 4: Research the fish before you fall in love with them

   Adult size, temperament, whether they shoal, water parameters and compatibility. The common failures are predictable: common plecos reaching 45cm, goldfish needing far more volume than a bowl, and single specimens of species that are miserable in fewer than six.

   You will know it worked when: A stocking list that will still work in two years.

5. 5

   Step 5: Add fish slowly, a few at a time

   Each addition increases the bioload and the bacteria need days to catch up. Two or three weeks between additions, testing as you go.

   You will know it worked when: The filter keeps pace with the fish.

Fishkeeping equipment is sold as a starter kit and is almost never the right starter kit. The two things that determine whether a tank succeeds — volume and filtration — are the two things kits economise on, because they are the two things a display box cannot show.

What actually matters, in order:

• The tank, and specifically its volume — Bigger is easier. This is the single most counterintuitive thing about the hobby and the most important: a larger volume of water dilutes waste, buffers temperature and pH swings, and gives you hours to notice a problem rather than minutes. A 60-litre tank is a reasonable first tank; a 20-litre desktop cube is an expert’s tank sold to beginners. Footprint matters more than height for most fish, and a full tank weighs roughly a kilo per litre plus substrate, so check what it is standing on.
• A filter, rated generously — The filter is not primarily a strainer — it is the surface on which the bacteria that process ammonia live. Rated turnover of four to six times the tank volume per hour is a sensible starting point. Never wash filter media in tap water: chlorine kills the colony you spent a month growing. Rinse it in old tank water at a water change, and only when flow drops.
• A liquid test kit — Ammonia, nitrite, nitrate and pH. Strips are convenient and imprecise at exactly the concentrations that matter. Without a test kit you cannot cycle a tank, cannot diagnose a problem, and are keeping fish by guesswork — this is the purchase people skip and then regret.
• A heater with a separate thermometer — For tropical species. The thermometer is not redundant: a failed heater stuck on is one of the few faults that kills an entire tank in a day, and the dial on the heater tells you what it was asked to do, not what it did.
• Dechlorinator — A few pounds, and required for every water change on mains water. Chlorine and chloramine are added to make water safe for people and are lethal to fish and to the filter bacteria.
• A siphon and a bucket — The gravel vacuum is the actual maintenance tool of the hobby. A bucket used for the tank should be used for nothing else, ever — detergent residue is fatal.
• Lighting and substrate — Lower priority than the list above unless you want live plants, in which case they become the constraint. Ordinary gravel or sand is fine for a fish-first tank.

What you can skip at the start: an air pump, unless a specific fish or filter needs one; chemical pH adjusters, which cause more instability than they cure; and almost every bottled additive other than dechlorinator. Stability beats optimisation, and most additives are sold against a problem that good water changes solve.

If you learn one thing before buying anything, learn this. It explains most beginner fish deaths and it is the reason experienced keepers seem strangely uninterested in which fish you want.

Fish excrete ammonia, which is highly toxic to them even at low concentrations. In an established tank, colonies of bacteria living in the filter media and on surfaces convert ammonia to nitrite — also toxic — and a second population converts nitrite to nitrate, which is far less harmful and is removed by water changes and taken up by plants.

A brand-new tank has none of those bacteria. Water conditioner does not supply them and a filter does not contain them out of the box. Establishing the colonies takes weeks, and until they exist, ammonia accumulates. Adding a full stock of fish to a new tank means they live in their own waste while the bacteria slowly catch up — which is what "new tank syndrome" describes, and it is the commonest way a beginner’s fish die.

The two ways round it:

1. 1

   Step 1: Fishless cycling — the better method

   Set the tank up fully, then dose ammonia — as pure household ammonia with no additives, or by letting a small amount of fish food decay — and test the water regularly. Ammonia rises, then falls as nitrite rises, then nitrite falls as nitrate appears. When the tank processes a dose of ammonia to zero ammonia and zero nitrite within about 24 hours, it is cycled.

   You will know it worked when: A filter that can handle waste before any animal depends on it. Typically three to six weeks.

2. 2

   Step 2: Seeding from an established tank

   Mature filter media, substrate or a sponge from a healthy established tank carries the bacteria with it and can shorten the process from weeks to days. This is what the hobby’s social culture is genuinely useful for — a local club or a keeper on a forum will usually hand over a squeezed sponge for nothing.

   You will know it worked when: A substantial head start, and the reason it is worth knowing another fishkeeper.

1. 1

   Step 1: Cycling the tank

   Establishing the bacteria that process ammonia before fish arrive. This is not a step to rush and it is the single commonest cause of early deaths.

   You will know it worked when: Ammonia and nitrite read zero while nitrate rises.

2. 2

   Step 2: A stable community

   Water changes on a schedule, feeding less than feels right, and stocking within the tank's actual capacity.

   You will know it worked when: Parameters stay steady for a month without intervention.

3. 3

   Step 3: Reading the tank rather than testing it

   Recognising trouble from fish behaviour, algae patterns and plant growth before a test kit tells you.

   You will know it worked when: You notice a problem before it becomes visible damage.

4. 4

   Step 4: The plateau

   Algae, universally. It is a symptom of an imbalance in light, nutrients and plant mass rather than a thing to kill, and people stall by treating it as an enemy.

5. 5

   Step 5: Where it branches

   Planted aquascaping, breeding, species specialisation, or larger and more demanding systems. Breeding changes the hobby from keeping to husbandry.

6. 6

   Step 6: When to upgrade

   Bigger is genuinely easier — larger volumes are more stable. The upgrade trigger is usually stocking ambition rather than dissatisfaction.

• Partial water change — Typically 20–30 per cent weekly, matched for temperature and dechlorinated. This is the core of maintenance: it removes nitrate and replenishes trace minerals. Never change all the water — you would remove the bacterial population and the stability with it.
• Siphon the substrate — Done at the same time as the water change with a gravel vacuum, pulling out the detritus that would otherwise decay.
• Rinse filter media in tank water, occasionally — And never under the tap. Chlorine kills the bacterial colony living in the media, which is the entire filter. Rinse gently in the water you have just siphoned out.
• Test — Weekly at first, less often once a tank is established and stable. Ammonia and nitrite should read zero; a reading above zero in an established tank means something has gone wrong and is worth investigating immediately.
• Feed less than you want to — Overfeeding is the second commonest beginner error after skipping the cycle. Uneaten food decays into ammonia. What the fish clear in a couple of minutes, once or twice a day, is enough — and most fish are perfectly well on a fast day each week.

Fishkeeping splits first by water chemistry and then by intent, and the first split is the one that decides cost, difficulty and which advice applies to you. Freshwater and marine are close to different hobbies sharing a word: the equipment, the livestock supply chain, the failure modes and the running costs all differ substantially.

• Community freshwater — Mixed peaceful species in a planted or decorated tank. The default entry point, the cheapest, and the most forgiving of the mistakes everybody makes in the first six months.
• Species and biotope tanks — One species, or a set that would genuinely occur together in one place, with matching water parameters and hardscape. More restraint and more research, and generally healthier fish for it.
• Planted tanks and aquascaping — Where the plants are the point and the fish are the accent. It becomes its own discipline quickly — carbon dioxide injection, substrate, lighting and trimming schedules — and is published here as a separate subject.
• Coldwater and ponds — Goldfish properly housed, native temperate species, and outdoor ponds. Cheaper to run, larger volumes, and a seasonal rhythm rather than a constant one.
• Marine, fish-only — Salt water without corals. Higher equipment cost, salinity to manage, and a much less forgiving margin when something goes wrong, because the livestock is more expensive and often less robust.
• Reef — Corals as well as fish, and the most demanding branch by a wide margin — lighting, flow, and a list of water parameters that must stay stable rather than merely acceptable. Also the most spectacular, which is why people attempt it too early.
• Breeding — A cross-cutting specialism rather than a tank type. Livebearers to start, then egg-layers, then the species with genuine difficulty. It changes what the hobby is about: you end up managing several tanks and thinking in generations.

Cheapest branch to start

Community freshwaterAnd the one where a mistake costs the least, which matters more than it sounds.

Biggest jump in cost

Freshwater to marineRoughly a doubling before livestock, and the running costs rise too.

Where the difficulty actually is

Stability, not complexityReef keeping is hard because parameters must stay steady, not because any one of them is obscure.

The branch that changes the hobby

BreedingIt turns one tank into several and shifts the interest from display to husbandry.