How to set up a fish tank
Set the tank up and let it run empty for at least a week before adding any fish — this is the single most important step, because it lets the water cycle and the filter build up the bacteria that keep ammonia from building up to dangerous levels once fish are actually living in it. Skipping this step, more than any single piece of equipment, is what causes most new tanks to lose fish in the first few weeks.
Position, size and substrate first
Choose a spot away from direct sunlight and radiators, since both cause algae growth and unstable temperatures, and make sure the surface underneath can genuinely take the weight — a full tank is far heavier than an empty one looks. Bigger tanks are generally more forgiving of small mistakes than very small ones, because water quality changes more slowly in a larger volume.
Rinse gravel or substrate thoroughly before adding it, since dust from new substrate clouds the water and can clog a filter almost immediately. Add substrate before water, then decorations, arranging anything a fish could hide behind before the tank is full, which is far easier than trying to move things around once it is.
Filling, filtering and the nitrogen cycle
Fill the tank with dechlorinated water — tap water almost always needs a conditioner to remove chlorine and chloramine, which are safe for people but harmful to fish. Fit and run the filter from day one, since this is what will eventually house the beneficial bacteria the whole system depends on.
The tank then needs to 'cycle': beneficial bacteria build up in the filter media over one to several weeks, converting the ammonia that would otherwise come from fish waste into less harmful compounds and eventually into nitrate, which is removed through regular partial water changes. Adding fish before this bacteria colony has established is the single most common reason a new tank runs into trouble.
Heating, lighting and testing before fish go in
Match the heater and lighting to the species you actually plan to keep, since tropical fish need a stable, warmer temperature while some coldwater species do not need a heater at all. Research your intended fish before buying any equipment, not after, so you are not retrofitting the tank around fish that are already in it.
Test the water with a proper test kit rather than judging by eye — clear water is not the same as safe water, and ammonia and nitrite can both be at dangerous levels in water that looks perfectly clean. Only add fish once ammonia and nitrite are both reading at zero.
Adding fish gradually and keeping the tank stable
Add fish a few at a time rather than all at once, even once the tank has cycled, since each new addition briefly increases the bioload the filter has to cope with. Float the bag the fish arrive in for around fifteen to twenty minutes to match the water temperature before releasing them.
After that, a regular routine of partial water changes, filter maintenance that does not involve replacing all the media at once, and consistent feeding keeps the system stable. Most ongoing problems come from skipping this routine maintenance rather than from anything wrong with the initial setup.
Common questions
- How long should a fish tank run before adding fish?
- At least a week, and ideally longer, to let the tank begin cycling before any fish are added. Testing the water with a proper kit, rather than going by a fixed number of days, is the more reliable way to know the tank is actually ready.
- Do I need to treat tap water for a fish tank?
- Yes, in almost all UK areas. Tap water contains chlorine or chloramine that is safe for people but harmful to fish, so a water conditioner is needed every time you fill or top up the tank, not just at the initial setup.
- What is the nitrogen cycle in a fish tank?
- It is the process by which beneficial bacteria in the filter convert ammonia from fish waste into less harmful compounds and eventually nitrate, which is removed through water changes. This bacteria colony needs time to establish, which is why a new tank should run and be tested before fish are added.
- How many fish can I add at once to a new tank?
- Only a few, even in a tank that has already cycled, since each addition increases the waste the filter has to process. Adding fish gradually over several weeks gives the bacteria colony time to keep pace, rather than overwhelming it all at once.
- Do all fish tanks need a heater?
- No — it depends entirely on the species you plan to keep. Tropical fish need a stable, warmer temperature and require a heater, while some coldwater species do not. Research the specific fish you intend to keep before buying equipment, rather than assuming one setup suits every species.
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Written against guidance published by UK veterinary and welfare organisations. Always confirm specifics with your own vet.
Written by Matt Garnett, who runs Giddy Pets and is a lifelong pet owner — not a vet. This is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your animal. How we write these guides.