What's wrong with this tank?

This is a 10 gallon tank that I have been using off and on (mostly on) for the past five years. I've raised many batches of mbuna fry and tons of bristlenose plecos and Julie fry were the most recent inhabitants that grew out in it.

I have always rinsed the tank out well with water, let it dry and kept it in the same location until its next use. It's always been fine -- no problems -- until last month or so. I bought several small fish from a store in Portland, OR and put them in the tank for quarantine. Used the same equipment and a box filter that I keep in a healthy pleco tank, dechlorinated the water, acclimated them the way I normally do and the next morning all 20 fish were dead. I figured it was something to do with the stress of traveling and the new tank, etc. although I'd never had anything like this happen before and didn't do anything differently.

I rinsed the tank and equipment well, let it dry outside in the sun and put it away until a couple of weeks ago. I put about 30 L. caeruleus fry in that tank along with mostly water from the tank they came out of. All dead the next morning. Now I started to suspect something had contaminated the tank, although I can't figure out what it could possibly be. I rinsed it out over and over, added a little bleach and let it soak for a day, rinsed it and soaked it more, and let it dry in the sun for a few days. I made sure that nothing could contaminate it.

Yesterday I needed to move some young leleupi temporarily, so I pulled the tank out and rinsed it a few more times, added water from the leleupi tank along with their box filter and a few hours later they were all dead!

This tank is now going into the trash! I'll never use it again.

I have no idea what happened.

Wow. Sorry to hear about your troubles. Something non water-soluble must have gotten into it. I can't imagine what it could have been but I definitely agree with trashing it.

I can't imagine what happened, either, Betty. So sorry it caused you to lose so many fish, though.

That is terrible . When it happens this fast it must be something poisonous. Have you resealed the tank with new silicone before this started to happen? If so you might have picked up anti mould silicone and that kills of fish pretty fast. I know of silicone makers that has put this into aquarium silicone tubes by mistake.

No on the resealing. Nothing has been done to the tank and I just can't think of a thing that might have come into contact with it or the equipment/decor that stays with it.

I wonder if the heater might have somehow zapped them. I've had my hand in the tank and not felt anything, so I don't think that's it, but I'm afraid to use anything that was in the tank now. When the fish (the first two groups) went in the tank each time, they seemed perfectly fine for awhile, swimming and exploring and were just dead in the morning.

The leleupis went in earlier in the day and later that day some were dead and a few others were twitching and swimming erratically, very near death.

The heater very well could be shorting out. Even if you couldn't feel it, I bet the fish could. Hubby wouldn't happen to have a multimeter would he? Don't throw the tank away just yet, lets do some experimenting first. This could get interesting.

Sorry there were so many losses

Yeah, he does somewhere...

i was going to say heater also, i have seen heaters do things like this before.

Have him dig it out for ya, pm me if you find it and I'll help ya test it

He's out of town, but I'll find out where it is and let you know. Thanks!