I’ve got a 45 gallon fishtank with a single 4’ florescent bulb over it.
I recently added a half-dozen platies and two snails and all but one platy slowly died over about three weeks. The snails went first, FWIW.
I took some water to the store and they suggested that my Nitrates were a bit high and I should change what chemical I’m using to pre-treat with my water changes (from Aquasafe to Prime) and that I add some salt to the water (I had none before).
Perhaps related, perhaps not, I seem to have a pretty strong algae layer growing now. I scrubbed the glass yesterday, but the plastic plants (no live plants) are still pretty heavily covered and the inside of the tubes from the under-gravel filter to the power heads are layered, too.