put activated carbon in your filter and the smell will be absorbed... alternatively, make sure theres no decomposing plant matter, give the tank a really good clean... or check in the hood and cover glass if some food has landed where it cant be seen and is smelling up the place
I cut the leaves as soon as they start turning brown to prevent that and the food I feed the fish low amounts so there's little to no waste food but three times a day to make sure they eat enough.
I'll have to go the carbon route seeing as it's been recommended more than once.