Mystery deaths.

[k1rr1e]

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?

[k1rr1e]

I should have added that many of my fish were obviously struggling for oxygen inspite of the fact that I have a diffuser on the filter / pump outlet. Possibly why the Gourami's are all fine, but other fish have succumbed.

I had also applied a dose of interpet Anti-fungus treatment to treat one Cardinal, which has unbelievably survived, although the fungus is still there.

[Carylnz]

Have you checked the temperature? It is possible the heaterstat is faulty.
The water changes are good. Changing all the filter elements (do you mean media?) is not although you say the water tests are normal.

[k1rr1e]

The temp seems to be fine, the heaterstat is set to 26C and I have a thermometer in the tank as well, to check the actual water temp.

Poor terminology from me regarding the filter. What I should have said was that I changed only one of the three filter foam media for a new one; I only ever do one at a time. However, I have stripped down the pump / filter unit three times now and cleaned every part. Each time I do this, I clean each of the filter foams in tank water.

What might be the original cause of the problem, did the new foam media element upset the balance of the tank? Did the anti-fungal treatment deplete the oxygen levels in the water too much? Should I have used an additional air pump (which I don't have at the moment) to provide additional oxygen whilst treating the fungus?

Questions, questions?

[Carylnz]

Assuming this tank has been running for quite some time so fully cycled and matured, I can't see how changing one of the filter sponges could cause this problem.
As I do not use medications in my tank I am unable to comment but have heard some medications do lower saturated oxygen levels so this may have contributed.
Often, in cases like this, it is a combination of factors rather than one single cause.
I would be curious to know the cause of the fungus. If it only affected one fish I would have removed it and treated it separately rather than dose the whole tank.
Sorry I can't be of more help but there are so many factors that could be affecting things it is hard to say whether it will happen again.