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I have lost a few fish that originated in a particular tank - thinking that it was my own fault for the most part but the deaths seemed pretty unexplainable.

The original holding tank where the fish were for about 3-6 months was a 20g with an AC 200 , temp 82-84 , PH7.8, GH 11 (tapwater) no additives except declorinator. I don't believe I have used meds except melafix. Weekly 30%-50% waterchanges or more.
There is a piece of driftwood and bare bottom.

The fish in the tank were originally 3 albino ancistrus, 3 L-264 and 6 geo redhead tapajos.
2 of the L-264 died the first week so there was just 4 plecos at 3" each and 6 small geos <2".

Everything was fine for about 5 months but then suddenly one geo died (?).
Couple of weeks later I moved the albino ancistrus to their own 30g (well cycled) with fine gravel , driftwood and a few flowerpot caves to try to induce spawning.

Over the last 3 weeks I have noticed the fish in the original tank breathing very rapidly and have lost 3 more geos on different occasions - I have increased waterchanges to 2-3 times per week and cleaned the filter out a few times (in tankwater) - the fish don't seem to have any injuries of any presence of parasites on the gills ( though I don't know what to look for) even with a magnifying glass. The stomach is just goo - no internal organs left recognizable - I don't know if it decomposes that quickly since the fish would have been dead for less then 24 hours. The last 2 guys are breathing very very rapidly now and they do so after waterchanges also.

In the ancistrus tank I did many large waterchanges and it has a powerhead with venture plus a hydrosponge. I noticed that the fish were in the surface the last few days and have done several (daily ) waterchanges. Their fins are looking ragged and this morning the female was dead. The 2 males are breathing heavily.

I feed moderatly with mixes of stuff tailored for the particular fish/tank (dry - frozen - live) the food and the tapwater used for those tanks is used on many other tanks where there is no problems.

I have checked the waterparameters and gotten PH 7.6-7.8, Gh 10-12, NH3/4 0, NO2 0, NO3 0.

I am bummed out about the events and have no idea what could be wrong or even what else to look for. All I can imagine is that there is some sort of disease/parasite which was harbored by one of the fish in the original tank and has slowly taken hold and is now just exploding.

I did medicate with metronidazole when the plecos first came for a week and all the fish have been eating well and acted normally until the last month.

As I have used the same buckets for numerous other tanks without the "thing" spreading it seems that it has to get physically transfered.
I am wondering if I should just clean the tanks and sterilize and start them over or keep looking out for more clues.

I'll be thankful for any help.

What type of filtration are you running? How big is the tank?

The gasping is sometimes a sign of the water not having enough DO (Dissolved O2). Having enough surface movement (powerhead, water bubles, etc.) will help oxygenate the water. The fasping could also be a sign of too much ammonia. Performing such a large water change on a bare bottom tank is, IMO, risky. With no gravel substrate, the bacteria you're relying is contained on the driftwood and on the water. Since you removed most of the water during the water change, the bacteria would need time to catchup to the load of the tank. Therefore, you may have a several spikes in ammonia.

You mentioned the stomachs looking like goo after they died...did the fish show a large stomach at all? If so and if there scales (if any) appeared to be sticking out, this could be a sign of dropsy.

Thanks CE,
as I mentioned in the initial post the tanks are 20 and 30 g with an AC200 HOB on the 20 and a PH with venture + aquasponge with air on the 30. IMO the fishload is moderate on the 20 and very low on the 30g. The 30 also has gravel. I mentioned the readings in the previous post as I usually do them when there is sign of problems.

There was no obvious sign of problems with the fish - no swollen stomach or mucus on gills. The last 2 geos are breathing a bit easier today (70% waterchange this morning) the L-264 doesn't seem affected.

In the 30g however both the surviving albino ancistrus are at the powerhead near the surface.
The biggest bummer is that the female that died laid eggs and there are a few scattered about and a small cluster in one of the caves.

Sorry, I didn't look clearly with all your acronyms

Anyway, one last question, when was the last time you used melafix? What were you treating?

I used Melafix yesterday , before that I can't recall. I'm trying to figure out what would make these 2 tanks different from others I have - since water- chemistry, changes , parameters and temperature are same or similar for a number of tanks I have as is feeding.

On the ancistrus tank I did daily watechanges on a daily basis for a week to try to induce spawning.

One thing I can think of is I tried the "Waterpurifyer" from AP - a small DI unit and stored the water in a new Rubbermaid bin which I had rinsed out several times with water. I used it about a week ago and added the water to several tanks - the ancistrus tank I know and a few others.

I haven't seen any problems when testing the water and it really hasn't changed the chemistry much - maybe a fraction on the PH (lower) and a degree or two on the hardness.

I'm really stumped - I wish it would be something simple mechanical that I could fix easily but at this point I'm guessing something with disease or parasites that is complicated and over my head.
I lost another geo overnight - eventhough I'm doing large waterchanges and the ancistrus tank isn't looking too good either.