Hi there,
I have suffered a number of mystery deaths recently, which I cannot easily explain.
I have a mature community 120l tank with an Eheim aquaball 130 internal pump / filter. I recently changed one of the three filter elements and it maybe coincidence, but that was when the problems seemed to start.
In the two weeks since changing the element I have lost nearly 50% of my fish (4 Cardinals, 3 Rams, 1 Odessa Barb, 1 Swordtail and several shrimps) despite daily smaller and larger water changes and cleaning every element of the filter system several times. The only species not affected are my Gourami's (Pearl, Golden and Dwarf) and Sterba's Cory's.
Water tests were normal apart from a slightly high nitrate level initially, but I lost more fish after the nitrate level had returned to normal.
The only thing I have noticed is that the Pump / Filter which is about 18 months old, seems to require cleaning much more often. I have toyed with buying an external pump / filter, but would welcome views and ideas on the possible cause of the above and to prevent a reoccurrence?