Tank crash & recovery - next steps?

[Doug Osborn]

Hi All,

I have a 850l planted discus/community tank. I ran the tank for about 12 mo before introducing 8 adult discus 6 mo ago. They soon coloured up, grew, and formed 3 pairs each of which spawned. 1 pair was very regular (every week or so), the others less so (4-5 weeks).

The other species seemed to be doing very well too, with the sterbai cats and black banded rainbow fish regularly spawning, and even glass catfish filling with roe and doing a writhing dance to scatter eggs everywhere.

The tank had never had any detectable ammonia, nitrates often 0 but sometimes with 5-10 readings, and no nitrites.

In mid-Nov the heater stuck, leading to a temperature spike (which the discus loved at the time, but killed some other fish and I suspect snails and bacteria) and ammonia spike (2.0 ppm was the highest reading I had).

This lead the discus to become incredibly skittish and they self-inflicted some horrendous injuries. I've just euthanised my favourite, a large blue diamond male (one of the prodigiously spawning pair) who'd ripped off a pelvic fin and obviously did some internal injuries as well.

The tank took me weeks to restablize, with the sterbai cats recommencing spawning again a couple of weeks ago, and 1 pair of discus last weekend. All the remaining discus are back to full colour and behaviour, confidently swimming up whenever someone nears the tank.

I'm left with many questions but the key ones are:

- What caused the ammonia spike following the stuck heater where I've never had any before? The heat killed a few fish, but was it the (dead) biomass of the snails and bacteria colonies in the substrate?
- Why was the bacteria colony so ineffective at removing ammonia? It seemed to 'digest' ammonia at low readings (0.25) but at higher than that, the only reduction was through water changes. Was it killed by the heat?
- What can I do to prevent this in the future? Add a fluidized bed filter?

Many thanks,
Doug

[taksan]

The dead biomass of the snails caused a temporary massive spike in ammonia that was too much for the established bacteria to handle. Heat won't kill the bacteria the discus would die first. but a sudden large number of dead snails rotting away would be too much for a filter used to a certain load to handle.

[ILLUSN]

taksan is 100% correct, your filter could not handel the sudden surge of ammonia your luck not to have lost all your fish.

at levels above 2ppm ammonia becomes toxic to the filter bacteria, too much ammonia will kil the filter, also you get a huge shift in the bacterial pop from nitrite consumption species to ammonia consumption species, often its not the ammonia spike that kills discus (in in acidic discus water NH4+ ammonium is suprisingly less toxic then you'd think) but the follow on nitrite spike.

a FBF would help as they have a massive capacity to handel sudden increases in bioload but even a fbf might not be enough to remove a sudden influx of ammonia in one pass. even if it could your water coming out might be so anerobic that the fish suffocate due to no disolved oxygen.

you could hook your heater up to an external thermostat to shut it down at a preset temp.