New Tank, Established Filter, Fish Still Dying

[y2aak]

This is a long one, but I don't know which details are important and which aren't so here's the whole thing:

In a 5.5 gallon tank I wanted to use as a plant quarantine tank, I had some old eco-complete and several plants taken from another tank suffering from a midge fly infestation. I wanted some fish in the tank to eat any larvae or eggs that might still be on the plants, so I got three harlequin rasboras from the LFS. I acclimated them by placing them in a tupperware with some of the LFS water and adding tank water at a rate of about two drops per second. Once the fish were swimming in about two liters of water, I stopped the drip and let them swim around for about an hour. Then I put them in the tank. About six hours later, they all looked sick. By the morning, they were dead. The filter on the tank was an established filter, so ammonia and nitrite were at 0ppm and nitrate was 5-10ppm. At the LFS, I was told the tank temperature was 75 degrees, which is the same temperature my tank was at.

I threw out the eco-complete and the plants, rinsed the tank with water (no detergents or bleach), added fresh, rinsed gravel, and filled the tank with tap water treated with Prime. I also replaced the sponge and the carbon in the filter, but left the bio-media untouched. I got three more rasboras, followed the same acclimation procedure, and although they lasted about 8 hours longer this time, they still all died.

Any ideas as to what could be going on here? Could there be something in the filter that is leaching out, or could the midge larvae be releasing something harmful? I'm at a total loss to explain this.

 

[Smells a bit fishy]

Sorry, can I just double check that the Tupperware pot was in the tank? Also why not just use the bags? If you just plonk the bag in there, use drip acclimatisation you should be fine. If after 6-8 hours I would say its how you are adding the fish, although I could be wrong.

I wouldnt say it was the migde larvae either, as if this was the case, no fish would survive in the wild. Apart from how you acclimatise I can't think of anything else. However I'm sure a more experienced person will be along shortly

 

[y2aak]

The tupperware was not in the tank - they were just sitting next to each other on the same table. I also think acclimatization is the prime suspect, but I'm just not sure how what I'm doing is wrong.

 

[Harlequins]

I would say its how you acclimatised them...

The easiest way way to leave the fish in the bag,pop the bag into your tank then add a little tank water every 5 mins for about 30-45 mins -Then net them out of the bag into the tank.
The bag needs to be in the tank so they can acclimatise to the temp of the tank etc.

I may be wrong but you adding of water was far to fast,it should be slower than that so the fish can adjust to the different water/ph/temp.

Harlequins are quite a hardy fish,but shouldn't be kept in a 5.5gal tank,far to small,plus they are schooling fish,i think the stress of how you acclimatised them and putting them in a small tank added to their demise...