6 stupid fishkeeping mistakes

1. Not testing your aquarium water
Unfortunately you can't tell much about the chemistry of your water just by looking at it, or by merely looking at your fish. After all, vodka is clear, but you can't keep fish in it. A set of aquarium test kits should be considered an essential purchase for any fishkeeper.

You'll need ammonia, nitrite, nitrate and pH kits, but you can save money by getting a master test kit which includes reagents for all four. Although this could set you back £20, it will last you a year and it's going to save you money in the long-run.

2. Failing to check the suitability of new fish
There are literally thousands of different fish species available at aquarium shops, and new species and varieties are introduced into the hobby every year. Sadly, some of the shops importing and selling fish don't put a great deal of thought into their suitability for customers' fish tanks, so unsuitable species are commonly seen for sale.

Some might be aggressive, difficult to keep, have specialist dietary requirements or simply grow to ridiculous proportions. It's therefore really important that you double-check the suitability of any new fish or inverts you want to add to your tank before parting with your cash.

3. Not allowing for adult size
Most of the fish on sale in the shops are youngsters and will generally at least double in size, if not quadruple. You need to make sure that your tank has sufficient space to house them comfortably when they reach their adult size. Fish can grow very quickly, so unless you carefully consider their needs you could be lumbered with some fish that require rehoming in larger fish tanks.

4. Overstocking
There's a limit to the number of fish you can keep in an aquarium and fishkeepers typically use stocking density guidelines to help them determine this. Not only do you need to ensure you keep your tank's stocking below the recommended maximum, you also need to ensure that you'll still be below this limit when the fish reach their adult size. Using the expected adult lengths of the fish is generally the safest way to ensure you don't slip up.

5. Cycling with fish
When you start a new aquarium it's vital that you build up a population of friendly filter bacteria in the system before you add the fish. Without enough of these microbes the wastes from the fish will quickly build up, resulting in potentially dangerous levels of ammonia and nitrite and some poisoned, diseased or dead fish.

In the dark ages of fishkeeping people used to set up an aquarium and add a few "hardy fish" which would do their business in the water and help the bacteria to grow. However, this knowingly exposes them to toxins, which isn't particularly ethical. Nowadays, most experts will recommend you adopt the more modern fishless cycling technique instead.

Unfortunately, lots of aquarium supplies stores still dish out the dodgy old-fashioned advice, so be careful not to fall into the trap of following it. Find a decent fishkeeping forum, like the one on the Practical Fishkeeping site, and follow the advice of other fishkeepers on cycling your tank the modern way - without fish.

6. Washing filter media under the tap
This is perhaps the most common error, partly because some filter manufacturers continue to provide dodgy advice in their product instructions. Aquarium filters work biologically and rely on friendly beneficial bacteria to remove toxins from the water. However, tapwater contains chlorine and chloramine, which are added to kill them, so if you wash your filter under the tap (or replace old media with new) you'll inadvertently destroy them and poor water conditions will follow.

Instead, use a bucket of old aquarium water to wash your filter media in, and take care when replacing expired media. If you can chop the old media and new media in half and replace one bit at a time.

Mark Cropper works for Swell UK. Swell UK.com is a leading online retailer who stocks a wide range of aquarium, pond and garden supplies such as pond pumps and garden furniture.