death tank? - long

["Doug" <DDunlop@quortech.com>]

I seem to have another plague situation on my hands.

My tank is 155 gallons (600 liters) and heavily planted.

The water is 78 degrees F, pH 7.3, conductivity 75-100uS, ammonia and nitrite do not register on my test kits (new reagents)I add plant fertilizer containing chelated metals, potassium sulphate and calcium carbonate daily to bring the conductivity to 100uS.

The inhabitants and time of habitation are:
2 algae eating shrimp - 6 months
2 Apistogramma Agasizzi red - 4 months
4 Apistogramma biteniata 2 months
1 Cardinal tetra - 3 years
6 cory cats - 3 years
1 Kuhli loach 3 years in this tank and 9 years in it's predecessor.

I feed frozen brine shrimp, frozen mysid shrimp, flakes, chopped earthworms, live baby and adult brine shrimp on alternate days.  I try to keep the fish well fed but not fat.

The problem:
I had 7 (now 3) Laeticara curviceps in the tank.  I have owned them for about two years and they had been in the tank about six months.  Shortly after a water change with RO water, a pair spawned.  They raised their fry for three days post free swimming.  I rescued none of the fry but planned on grabbing some next time.  The day the last fry disappeared I did my regular water change. The following day, there were two dead Laetacara curviceps in the front of the tank.  The fish look a lot alike but I am fairly certain that they were the breeders.  I took a good look at the rest of the fish in the tank and I saw no signs of distress.  All of the fish were eating and behaving normally.  In fact, two more Laetacara had layed eggs.  The following day, today, I found two more dead Laetacara.  This time I am less sure about whether they were the breeders.  They are certainly leaving some fine looking bodies behind.  The bodies look robust and show no apparent sign of illness. Just this Saturday, someone who was over remarked that the two fish guarding fry were the best looking Laetacara that he had ever seen.
I do have some clue to possible sources of disease.  Last friday, I bought some cardinal tetras that have been steadily dying in their quaranteen tank.  It is conceivable to me that I could have inadvertently transfered some type of pathogen from the obviously ill tetras to the heretofore healthy Laetacaras.

I also have a theory that my sand substrate could have gone anaerobic.  I have seen a few small bubbles rise from the substrate since the first deaths.  I assume that these bubbles would not be other than H2S.  I would have thought that I ought to see some symptoms of this other than sudden death of fish though.  Maybe the spawning fish were doing a lot of digging and were somehow exposed to a pocket of H2S?  The substrate is 3 inches of silica sand (#9 sandblasting sand) with substrate heating.

I am now worried about the other fish in the tank.  I am most concerned about the aggies, (who are also now acting like they ought to spawn soon) but I really would like to prevent any further deaths if I can.   

If anyone has any ideas and/or knows the correct solution, I would sure appreciate it.
Thanks,
Doug


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