Every aquarium goes through the Nitrogen cycle. Fish waste and extra food decomposes into ammonia, which bacteria convert into nitrites, which different bacteria convert into nitrates. This is why you need to add fish slowly at first to establish these colonies of bacteria. The end result, nitrates, are still poisonous to fish, although much less than ammonia, which is why you need to do one or two partial water changes a week to dilute the nitrates.
The trick is that these bound nitrates are perfect for plants, so water your house plants or garden with the water removed from your aquarium. Along with the plants getting fertilized, they'd get watered at all, which is easier to remember when it's taking care of the fish at the same time.