Yes Birds make one hell of a mess. Between the seed that is uneaten and or the empty shells. To the bird droppings, it all ends up in the tank. It's been over a year now and i haven't added a single fish . There are amano shrimp, kohli loch, yellow pelco's, otto cats, galaxy rasboras, red nose rummies, black tetras, goo oboo's they never show all at once making it impossible to count but let's just say there's no lack of activity in the tank. While over a year i'm sure i have lost a few fish here or there I just can't note any one FISH that is missing. As a matter of fact i did buy a " one" small striped pelco "sorry i for get it's name" and about a month ago i noticed there are now two of them and i found two baby striped pelco's. the only explanation i can come up with is a yellow and the striped pelco must have mated long ago and now it would appear they're breeding. i do 50% water change FAITHFULY every week. And after 16 months now no I'll effects to the tank from the birds. one last note i should make is that the water fall leaches through out the entire upper planted area which the fish waste that liquefies in the tank then fertilizes the plants which in turn the plants help clean the water. Now I'm no biologist nor a chemist it's just my guesstimate.