There are three things in fishkeeping that reek something awful. Two of them I deal with from time to time, the other doesn’t happen in my tanks.
#3 Stinky fishkeeping odor – Unmaintained tank. Whenever the water turns green, you have somebody too lazy or too busy to take care of it. The smell of green slime algae is pretty bad, but there’s an app for that. There are fish and snails and shrimp who eat algae, but only in controlled conditions. Out of control, algae will take over. When it does, wow, does it stink.
#2 Dead snail. Snails are great additions to your clean-up crew, and some species are very attractive in their own right. But, when you find a dead snail in your tank, take a whiff after you pull it out. You will only do this once in your life. It is baaaaaaaaaaad!
#1 The absolute worst fishkeeping smell of all is a microworm culture gone bad. Microworms are awesome food for fry, but consider this. The culture is a starchy food saturated with water, covered with yeast, and allowed to sit for weeks at a time. There are easy tricks to keep the little fruitflies away, but once that yeast and old oatmeal starts to go bad, the culture will drive people from your house. Cleaning out an old culture doesn’t take long, but you had better have a tolerant gag reflect, because this smell is absolutely nauseating.