Tank is 35 gallons, been set up since October.
Recently changed to sand substrate in late December, tank came through fine. The tank is moderately planted with Wisteria, stems, crypts, and some swords as the latest additions. Added new driftwood recently, kept an eye out for any fungus or anything and saw nothing, so I don't think that's the cause. Some plants are wild caught so there is a fear of pathogens, but they were cleaned well before addition to the tank and the latest additions were over a week ago.
Tank inhabitants are/were a Pearl Gourami pair, 8 harlequin rasbora, and 8 kuhli loach (4 adults and 4 juveniles).
On the first I lost one of my loaches. No symptoms, was eating fine and swimming around great the day before. I found him dead the next morning, nothing unusual about the body. Tank water tested normal, I thought it was just a freak incident.
However, ever since then most of my loaches have been hiding, which for them is not normal at all.
On the fourth I switched to a Fluval 206 canister. I went ahead and removed my old HOB filter as I figured since the bacteria is already established on the wood and sand, plus the plants, that I would be okay. Water has tested normal on the API Liquid test kit.
Today one of my female harlequin rasbora started swimming oddly during spawning (more than the regular flipping to lay the eggs under the leaves), and passed soon after. I was very upset and tested the water to check, and had normal readings (Ammonia 0, Nitrite 0, Nitrate 5).
Later on I noticed three more dead rasbora right before bedtime. No prior symptoms and nothing odd on the bodies.
I'm getting incredibly frustrated and upset and worried, as I have no idea what could possibly be happening.
It may also be worth mentioning that about 2 weeks ago I noticed white areas on the pupils of my female Pearl Gourami's eyes, and at first worried about a fungal infection but after some research I'm pretty positive that she scratched them on the dirftwood somehow, as the male will sometimes chase her towards the driftwood. It's been healing on it's own and she's showed no symptoms of stress or difficulty swimming or lack of appetite.
If anyone has any advice I'd really appreciate it. I'm honestly near tears. I'd really been thinking I was doing a pretty great job at taking care of these fish, and yet now...I have no idea.
There are pictures of the tank in my thread, Adventures of the 35 gallon if anyone would like to check them out. A few plants have been added since then.