i would slowly start ruling thing out by stopping the ferts and adding more air at night. dial back the co2 a bit as well. if they were fine before all this then something is causing it. they might be still being gassed at night just not as bad. loaches are more sensitive to things like ferts and meds being scaleless fish. even most meds say use at 1/2 dose for them so the ferts might just be to much for them. also what is the temp and nitrite.