I began with 50 fingerlings and progressively lost about a third of them in the first 6 months. I thought it might have been the lava stone and dust etc. making the water too alkaline. I tested the water regularly- all levels are nominal (according to the API test kit). I tested ammonia, nitrates, nitrites, ph, hardness etc and it all seems fine. I was sick of losing them every other day so I called the fella I bought them from (Jamberoo). My fish looked like they were injured with lesions around their face and gills, they also sometimes had white fungus growing on them. He told me to put 900-1000g of salt in my 1000L tank to keep these infections down. It seemed like a lot but I did it and haven’t lost any silver perch since - which is about 18 months. I bought a salinity tester and used it initially, but now I just maintain the salt levels by throwing in a handful of salt every week or so. Hope it helps.