When you put fish into an aquarium, you’ve crated a really small lake or ocean. Unlike the real thing, your lake has no predators, nothing but you to handle population control. It’s also a much smaller ecosystem than a real lake or ocean, which means that things can go wrong very quickly.
Not to panic – if you do regular water changes and remember not to overfeed, overclean or overstock your aquarium, you’ll have far fewer problems. Regular water changes are the housework of the aquarium fancy. If you don’t do them, it doesn’t really matter what else you do.
During our years in the pet store, we’ve met a lot of people who believe that topping up a tank to replace water lost by evaporation is the same as a water change. It isn’t the same at all. Imagine you’re in a small room and someone has just smoked a huge cigar. Also, everyone in that room has been breathing and producing carbon dioxide. Now suppose someone pushes some oxygen into the room without removing any smoke or any CO2. All that does is dilute the stuff you can’t breathe – it doesn’t improve the air quality a lot.
Fish produce ammonia when they breathe, and the breakdown of fish waste and uneaten food also produces ammonia. The aquarium contains bacteria that eat ammonia and excrete nitrites, which are less toxic than ammonia. There are also other bacteria which eat nitrites and excrete nitrates, less toxic again. But all those toxins are still in the water until you come along and remove some of the water – and some of the toxins – and replace it with clean water.
Here’s the other thing – changing half or more of the water every six months isn’t going to do a lot for your tank. Change 10% once a week, or 25% once a month. Pick one and do it. I’m a big fan of the 10% one. That’s a less-stressful change for the fish, and overall it changes more of the water.
Salt water, fresh water, tropical or cold water fish, they all need that basic maintenance. Do the housework.