Last week, we introduced six male guppies and a swordtail to our new tropical freshwater aquarium. Sadly, during the week, three guppies died. After much research, I learnt about the nitrogen cycle and how this will have led to the deaths of the fish.
I feel quite sad that I hadn't researched this myself before we bought the fish, leaving the 'fish homework' to my husband! However, in all fairness, he had done some reading but hadn't heard about the nitrogen cycle.
It is quite basic chemistry too, something as I biochemist I am quite ashamed to have forgotten about. Simply, uneaten fish food and fish faeces decompose and produce ammonia or ammonium depending on the pH of the water. Ammonia raises the pH and makes the water toxic for fish.
Over the course of about a week, bacteria grow that convert ammonia to nitrites. This compound is just as toxic to fish. After another week, different bacteria develop that convert nitrites to nitrates. Nitrates can be removed either by partial water changes or by live aquarium plants.
So, after two midweek partial water changes (about 15-20% replaced) our fish are much happier, so we decided to buy some replacements. Hopefully they will like their new tank mates.