It's often said that keeping fish, is more accurately described as a case of keeping water. All fish have certain requirements when it comes to water, some are very fussy, and can survive in only specific conditions, whilst others can survive in almost any water, at the right temperature. The majority of fish are somewhere in between, and will survive in most waters, but need specific conditions to breed.
The water must of course be kept clean, generally achieved through a mix of filtration, water changes, cleaner animals and plants.
There are several key parameters that your aquarium water must get right to ensure your species thrive. In general, most fish from your local fish shop will be living in your local tapwater, treated to remove chlorine and other additives, that water companies add. This conditions are easy enough to replicate, and will probably be perfectly adequate for these fish. This is particularly true of the cheaper species that make the bulk of your dealers stock. There are of course other fish that like very different water, and these will require specialist knowledge of their natural habitat; perhaps very soft and acidic, like a rainforest swamp, or hard and alkaline like the rift lakes of East Africa. Providing conditions for these species is easy with a little know-how.
When we talk of water quality in the aquarium, we generally mean the quantity of other substances mixed or dissolved into the water. These may have already been in the clean water you are using (typically inorganic carbonates, or 'hardness' in tap water); or the result of biological processes occurring within the tank (ammonia, nitrites and nitrates from fish poo, decaying food and plants, dead fish and snails). Wild fish don't live in 100% pure H2O, but providing water with 'acceptable' levels of other substances is critical.
Depending on where you live, you may use tap water, well water or collect your own rainwater. Tap water is generally treated with chlorine or chloramine by the water company, in order to kill bacteria, and make the water safe to drink. Unfortunately these chemicals will also kill your fish. Tapwater MUST be treated with a dechlorinator before adding to your tank. Luckily dechlorinators are inexpensive and readily available. But make sure you get one that removes both chlorine and chloramine. I use API tap water conditioner Read more about preparing tapwater for the aquarium here.
Although conditioned tap water is fine for most fish, there may be other undesirable dissolved chemicals, especially in areas with significant agricultural run-off. Otherwise clean tap water can be rich in nitrates and phosphates, and although not too harmful for most fish they are great fertilisers for algae. If you otherwise keep a clean tank, but struggle with algal outbreaks, it might be worth testing your water for nitrates.
If you intend to keep more sensitive species, or have significant algal problems, it might be worth investing in a reverse osmosis unit. This effectively converts your tapwater to (almost pure) H2O. You will have to add some salts back into the pure water, but you will have ultimate control over hardness, pH and dissolved impurities.
Ammonia is toxic to fish at high enough concentrations. Uneaten food, decaying leaves, dead fish, and fish excretion are all sources of ammonia. Removing this with a biological filter is essential. Actually the filter converts the ammonia first into nitrites, and ultimately nitrates. These nitrates have then to removed with water changes, otherwise they will simply feed an algal outbreak. Understanding the basics of the nitrogen cycle is an important part of fishkeeping.
Aquarium water test kits are really useful tools for the aquarist. They can alert you to problems before they get too serious: perhaps you filter is not functioning adequately, or your tapwater is overly nitrate-rich. Test strips are easy to use, inexpensive and can measure several parameters at once, with very little effort.
In a brand new fish tank, murky water is a common problem, but usually not serious. If aquarium water is cloudy it usually results from a bacterial bloom, generally because the filter is not yet fully established. This will usually sort itself out after a few days, as denitrifying bacteria colonise the filter. A second reason may be debris amongst new gravel. A small amount of filter wool in the filter will clean this very effectively.
Cloudy aquarium water in an established fish tank still results from a bacterial bloom. However, in a well stocked aquarium, this represents a far bigger problem than in a lightly-stocked new set-up. Again, this will result from a malfunctioning filter, perhaps in conjunction with overfeeding (don't) and/or overstocking (again, don't). Perform a large water change, and clean the filter. Reduce the amount of food you feed and/or the number of feedings.
Green fish tank water is simply an algal bloom. The water is most-likely too rich in nitrates, with too much sunlight. Again perform a large water change, reduce feedings and consider moving the tank away from windows.
Feed a varied diet Flake food is an excellent nutritionally balanced food for tropical fish, but all species relish a varied diet. Where appropriate, try feeding commercially bought frozen livefoods, very small pieces of shellfish (mussel, prawn etc) or culture your own Daphnia, brine shrimp, or worms. See more about foods and feeding here
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