Hi I'm new and having trouble manuvering on this site so someone suggested I try here. I have a freshwater 75 gal tank with a 20 gal sump. I was given the top tank from a neighbor who used it for saltwater. Had to buy everything for the the bottom sump and it was a DYI project. I killed my 7 year old parrotheads when I transferred them to this new big tank. The tank was cycled for a month and a half. The water parameters were fine. Something is wrong with the build, not sure what. I'll describe the sump starting from the left side. First section has the inflow from the tank and houses the filter sock and I had ceramic media in it with an over the top flow into the next small 3 inch section, followed by a under bottom flow into the next section which is also 3 inches that has an over the top flow into a large section which I had hoped one day to be a refugum. The last section is an over the top flow into the pump area which is the return to the top tank. The first section with the filter sock had a thick film of something floating on the top of the water. I had bought more small fish after the first killing and they all died within a day. two of the black moors got pop eye. Three time I have emptied out the whole system and cleaned it all out. Not I have bio balls and not sure what to do, so all my new fish are in a ten gallon tank for now. Please any help would be appreciated !!