oh and btw the shark got up and going within 2 minutes in the other tank. hes saved for now.

Are you sure a 2inch oscar can eat 1inch redtail shark, how big are the barbs-will they fit in the o's mouth. Have another look at your oscar mouth and see if the fish will fit in?

If you dont want to put all the fish in together, completely clean the tank with the bad water. Refill with some of the water from the good tank and the rest fresh. Then take some biological media in the filter from the good tank and put it in the bad tanks filter, and add your fish. You should find that the 'bad' tank will now be cycled/or will quickly cycle and the good tank will quickly recover from the missing media

i did just that. i didnt have the test kits so i dont know what went wrong. the "bad tank" was cycled ive had it for about 2 years without any problems. idk what the readings would have looked like then but right now i got a ph of 6.8 and nitrite of 0. which would almost absolutely mean no ammonia or nitrate.

as for the fish. they were in the basket and like i said would find a way out. the tiger barb made it into the oscars mouth completely twice and it spit him out with when i chased it with a net. the shark also got out but hid in the corner i think he woulda fit into the mouth of my os because they are skinny so i netted them and put them in a bucket. i finished the transfer and decided my best bet would be to put them into the tank of death.

the next morning, both fish were still alive and looking good, believe it or not. and i thought i saved them. today i found the shark dead i guess from all the stress he went through the past 2 days. he was laying on his side lifelessly, almost eaten, in a basket, a bucket, and back to his original home. tiger barbs looking great, surprisingly.

i thought for a bit maybe it was the oxygen. but water falls into the tank from the filter pretty harshly, making a decent amount of bubbles. and ive never had a problem like this in the past, even with much more fish. my bet is the ph dropped like crazy. but idk.

Sorry bout your loss.

If it was lack of oxygen the fish would of been gasping at the surface.

Was the filter working

Did you do anything different to usual

Did a fish die but you didnt find the body

How often did you change the water

I am just trying to think of reasons a perfectly healthy tank would quickly go real bad

This is a good example of why a test kit is a good idea. It may have given you an early warning things were going bad. Thus you can act sooner and may of saved the fish

no gasping at the surface.
filter was working.
nothing different then usual.
i did find the body.
water changed once a week. week and a half at the longest.
i had the kit, lost the tube. i ment to go out and buy it but put it off for 2 weeks and here was my results. ph was usually a tad low, but nothing life threatining.

i think i found the problem today though. i was looking through the gravel that i took out of the tank today and found 2 rusty pennies. i guess my little brother threw em in there. maybe while rusting they produced some sort of toxin and it finally got to them. but i thought the stress coat took out metallic substances in water. or maybe it took out enough to keep fish alive for awhile. they were very rusty and probably been there for over a year. what ya think?

Im not sure on this. I suppose the pennies could of leached some metals into the water but i doubt they would of rusted very quickly? Best to remove them tho and do some large water changes of the next week or so