Frequently asked questions on tank maintenance


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Matt Clarke answers some of the most frequently asked questions on tank maintenance.

Why do I need to do water changes?

Even in a well-filtered tank, the water gradually gets more polluted, and the chemistry alters the longer you leave the water in the tank. Nitrate and phosphate levels go up, and the pH level can drop. Partial water changes help to restore the correct conditions by removing wastes, diluting dissolved pollutants and restoring lost minerals.

How many water changes should I be making?

Expert opinions vary on the number of water changes you need to make. Technically, the number of water changes required depends on the number and size of the fish you are keeping, as well as the type of filtration installed.

Some tanks can get away with a monthly water change of 25%, but most need 25% each week to keep the water in prime condition.

A simple way to find out whether your current maintenance regime is keeping on top of pollution is to test the nitrate level in the tank and compare it to a sample of your tapwater. You cannot tell if you are changing enough water merely by looking at the tank or the fish.

If you are changing plenty of water, the nitrate level in the tank should be only slightly higher than the level in your tapwater. If the nitrate level of the tank is considerably higher than the tapwater, you'll need to change more water at each water change, or do small water changes more often. Increased phosphate levels, or a reduced pH in the tank, may also be a sign that you need to improve maintenance.

Alternatively, you can use my water change calculator to calculate the frequency and size of water changes your tank needs based on the nitrate increase.

How can I keep my gravel clean?

Each time you change the water use a gravel cleaning siphon to suck the dirt out of the substrate, rather than simply siphoning out the clean-looking water.

Gravel cleaning is particularly important if the tank is fitted with an undergravel filter, since the filter bed will block unless the dirt which collects there is removed.

There are lots of gravel cleaners on the market, but a basic 5-10 one will do the job as well as anything else. Battery or air-powered gravel cleaners are rarely as effective and can't usually be used to remove water.

What makes algae grow, and how can I get rid of it?

Algae feed on pollutants like nitrate and phosphate, which rise to high levels when there's lots of decaying organic material in the tank. Bright light (especially sunlight), and leaving the lights switched on for too long makes algae grow more quickly.

While there are lots of different chemical treatments on the market to kill algae, the most effective way to eradicate or prevent algae is to improve the water quality so it has less nutrients to feed on.

Test the phosphate and nitrate level of your tank and compare it to your tapwater. If the tank readings are higher, you'll need to step up the water changes, and consider adding some chemical filter media to help remove the pollutants.

If the tapwater is already high in pollutants (most is), it may be worth considering using Reverse Osmosis (RO) water. This is virtually free of pollution. However, it's also largely devoid of minerals, so you'll need to add a remineralising powder to it to make it safe for use in the tank. You can buy RO water from most good retailers, or invest in an RO unit to produce your own.

How can I unblock my undergravel?

The dirt that gets drawn into the gravel bed needs to be removed regularly with a gravel cleaning siphon to prevent the bed from blocking.

The bacteria which remove the pollutants in the fishes' wastes require lots of oxygen, and when the plate gets blocked the oxygen levels in the bed drop, making the bacteria less effective. If the oxygen flow to parts of the filter bed stops entirely because it's clogged with dirt, the bed will become anaerobic, and noxious gases could build up.

To keep the filter performing at the optimum levels remove the dirt before it builds up by using a gravel cleaner every week. Periodically, insert a hose down the uplift and siphon out any detritus that has accumulated beneath the filter plate.

How should I treat tapwater?

Tapwater contains added disinfectants, like chlorine and chloramine, which kill bacteria and other organisms making it safe for us to drink.

Unfortunately, both chlorine and especially chloramine are dangerous to fish and the essential bacteria that break down the fishes' wastes. Adding untreated tapwater can stress or kill fish, and quickly wipes out filter bacteria resulting in polluted conditions for several weeks afterwards.

Always wash filter media in a bucket of old tank water, never under the tap! Any tapwater added to the tank needs to be treated with a dechlorinator (water conditioner).

Check with your water authority to see if they add chloramine, as some are changing over from chlorine. Chloramine is a compound of chlorine and ammonia, and when it's neutralised by a standard dechlorinator it leaves the ammonia behind, which is not good.

While many treatments can get rid of the chlorine section of the chloramine, few get rid of the ammonia this leaves. Kent Ammonia Detox is one of the few products which removes both the chlorine and ammonia.

This article was first published in the Practical Fishkeeping health supplement, 2002.