The biggest risks to a tropical fish tank, and how to prevent them

1. Power cuts and load-shedding

What happens: the heater, filter and air pump all stop. Water cools, oxygen drops and, after a few hours, the bacteria inside a switched-off filter start to die.

• Keep a battery-powered or USB air pump (it can run off a power bank) for longer outages.
• Keep the lid on and wrap the tank in a blanket or towel to hold the heat in.
• Don't feed during an outage. Food adds waste the filter can't deal with.
• If a canister filter has been off for more than a few hours, rinse its media in tank water before restarting, so it doesn't push stale water into the tank.
• For valuable tanks, a small UPS or inverter for the filter is worth considering.

2. Heater failure

What happens: a heater stuck on can cook the tank. One that stops working lets it go cold, which is when white spot and other illness take hold.

• Use a separate thermometer and glance at it every day.
• Use the right size heater for the tank. On big tanks, two smaller heaters are safer than one big one.
• Replace heaters with cracked glass or loose seals straight away.

3. Hot summer days

What happens: tanks in sunny rooms or near windows can overheat on hot Cape Town summer days. Warm water holds less oxygen, so fish gasp at the surface.

• Keep tanks out of direct sunlight.
• On very hot days, open the lid a little, aim a fan across the surface and add extra aeration.
• Never drop ice straight into the tank. If you must cool it, float a sealed bottle of cold water.

4. Overfeeding

What happens: uneaten food rots into ammonia, clouds the water and feeds algae. It's the most common cause of poor water we see.

• Feed only what's eaten in about two minutes. Healthy adult fish are fine with one day a week without food.
• Holiday feeder blocks and automatic feeders often overfeed. Healthy adult fish usually manage a week without food better than a week of overfeeding.

5. New fish bringing in disease

What happens: one new fish can carry white spot or other illness into a healthy tank.

• Buy from a reputable shop and don't buy from a tank with sick or dead fish in it.
• If you can, keep new fish in a separate quarantine tank for two to four weeks first.
• Add a few fish at a time, not a whole group at once.

6. A neglected filter

What happens: the filter clogs, flow drops, less waste gets broken down and oxygen falls.

• Rinse filter sponges in old tank water during water changes. Never under the tap.
• Don't clean all the filter media and the gravel on the same day. Spread it out so the good bacteria survive.

7. Leaks and cracks

What happens: silicone ages, stands shift and glass gets knocked. A slow leak can damage floors and cabinets long before anyone notices.

• Check the stand is level, especially on tiles and older floors.
• Look along the seams for bubbles, peeling or white crusty lines.
• Keep heavy rocks off the bottom glass, or bed them on the sand.
• Check whether your household or business insurance covers water damage from an aquarium.

8. Sudden big changes

What happens: fish cope with gradual change, not shocks. Changing most of the water at once, a big temperature swing, or a dose of pH chemicals can all set off stress and illness.

• Change a smaller amount of water, more often.
• Match the temperature of new water to the tank.
• Fix problems slowly, over days rather than hours.

How regular maintenance lowers the risk

Almost all of these risks show warning signs before fish die: a heater reading a degree off, nitrate creeping up, a sponge clogging, a seam starting to lift. On every maintenance visit we test the water and check the heater, filter, pump, lights and seals, so small problems are dealt with before they become expensive ones.