Fish Suddenly Starting To Die Off Without A Visible Reason

[Chaotic-Crimson]

Tank size: 30 gallon
pH: 8.0
ammonia: Unknown (out of test strips and need to get more)
nitrite: 0
nitrate: 20
kH:80
gH: 180
tank temp: Around 80 F

Fish Symptoms (include full description including lesion, color, location, fish behavior): No lesions, lessening of color, or swimming in an odd location. Some acting very lethargic before dying the next day.

Volume and Frequency of water changes: 20% Weekly

Chemical Additives or Media in your tank: Aquarium salt (Very low levels and only added from when fish started showing signs of sickness, Water conditioner, and ammonia detoxifier.

Tank inhabitants: 1 bala shark(still less than an inch long, our petstore allows us to trade in fish once they get too large for their tanks).
List of what was there before they started dying: 1 balloon molly, 2 bala sharks.

Recent additions to your tank (living or decoration): None.

Exposure to chemicals: None.
 
So this started quite some time ago when I owned 3 balloon mollies (the one listed above was with them) and then two of them died leaving the black one as the survivor. I then isolated the survivor in a separate tank and took the main tank and rigorously cleaned it out without using any soaps of any kind, just hot water. I filled it up and let it get adjusted. I waited till the molly was acting normal and seemed healthy before putting her back into the main tank. I waited a couple days to see if the tank had any bad effects on the molly and once I thought she was all okay I went and got two bala sharks. 
 
I put the bala sharks into the isolation tank and waited a couple days for them to get adjusted and make sure they were all healthy, even treated them for ich. I then added them into the main tank with the molly. I made sure they were all okay and fed them regularly then after about a week one of the bala sharks died, no syptoms at all. He wasn't even moving lethargic. Then I moved the remaining bala shark to an isolation tank and kept the two separate waiting for them to show signs of sickness or die. After another week the molly suddenly died, same way. Perfectly active then suddenly dead.
 
Now I'm down to the last bala shark and he is still in the isolation tank. He has been very lethargic for about a day now. Just kinda floating around without trying to swim unless I feed him. Even then he eats less than normal. Right now he is just floating in one place and all of his fins are kinda twitching then he swims and bit and goes back to floating.
 
I don't think it's anything with the water otherwise they all would have died at once or relatively close together right? Is it possible the disease that killed my mollies before was still there and that's what is killing them?

 

[the_lock_man]

3 gallon? That wasn't a typo? I reckon that it's probably stress of living in a 3 gallon container that caused the deaths.
 
I would normally only recommend shrimp in something that small, but you have fish that need tanks 15 times bigger.

 

[Chaotic-Crimson]

I meant a 30 gallon my bad.
Update: I just ran another water change and he appears to be swimming around normally and actively but I am still concerned for him.  

 

[Lillefishy]

Bala sharks need to be kept in groups of at least 5, they stress easily & can be very skittish especially when kept alone, they also need a 6ft tank as they grow to over a foot & grow quite quickly given the right conditions

 

[the_lock_man]

OK, I see that you have edited your original post. In all honsety, I'm awfully glad it was a typo.
 
Mollies are a brackish species (but invariably sold as freshwater), and I never had any success keeping them when I was new to the hobby, but I always saw them going downhill for a week or two before they'd die.
 
With the bala shark, there's a few things, but they need to be in a group of 6 at the very least, and need a much bigger tank anyway. They also need very high quality water, they are very susecptible to a build up of organic waste. I don't think 20% water changes per week are really going to do it.
 
Since they should grow to 35cm long, to keep them correctly, you need a 2½m long tank, keeping them in a group of 6 at least, with filtration to match and a decent amount of water movement. Very few people can actually do that, so I would suggest you take the remaining individual back to your LFS, and swap it for something more suited to the tank and water conditions. With your high pH, you could go with some dwarf neon rainbowfish, or platies, or swordtails, or guppies.

Or you could change it completely and go with Rift Valley Cichlids.

 

[Chaotic-Crimson]

Thanks for the in depth suggestion lock man. I was planning on taking him back before he got too big but I will definitely be taking him back now and getting something else. Also just for note I had kept the mollies in fresh water for about two months before they suddenly went off the edge and some even had fry that I had raised and given over to the petshop.
 
Thanks again.

 

[Malex530]

First of all< Mollies can adapt to fresh brackish and even full marine conditions. But they are best in brackish water. In my experience mollies are sensitive to high nitrates. As well as The balas. Not to say that is the only source of the problem. 50% wter changes for 2-3 weeks and check your conditions after each one.