Below are the most recent pictures of the sharks for anyone who might have an idea as to whats wrong. Hopefully these help.
Sorry about the water on the glass haven't cleaned it off yet. Been more worried about the sharks.
Update: So I'm happy to let people know that the real problem why my sharks were becoming sick wasn't an issue with stress or foreign diseases it was a matter of not having the right water parameters. Something that petsmart should be shot for. They market these sharks as a purely freshwater species! This is NOT the case. After a certain point usually past 4 inches of growth these sharks need an infusion of brackish water to supplement their immune systems and keep them healthy and happy.
Also people need to be made aware that these sharks will get huge given the proper environment and should not be purchased by anyone incapable of keeping them in the proper tank. I was doing quite a bit of research on them to resolve their problems and found that in the wild these sharks often grow upwards of 3-5 feet each! As such you'll need a serious tank down the road to support them. Diet is also essential for these guys as they are more carnivorous than other fish. At one point we bought 12 additional small sword tails and zebra danio's to keep with them to liven up the tank and in the span of 30 minutes out running errands to our surprise all 12 of them were gone by the time we'd gotten back since Pavlo and Sauka had eaten them all lol.
They will require a very high quality diet of live fish supplemented by sinking carnivorous pellets (I recommend the excellent Hikari ones, they're expensive but your fish will love you for it and look positively stunning) and a varied diet of occasional algae wafers and greens as they are not strictly carnivorous. Its most important though that these fish have a tank big enough to support them. I would suggest nothing less than a wide and open 40 gallon breeder or larger tank for them as they are very active swimmers and very fast. Anything smaller and they have a tendancy to hurt themselves by banging into the glass.
Another thing they will also need are good water currents (mimiking the mango groves where they come from in south america) a nice rocky and sandy bottum, with a safe heater to keep the temps from getting too cold as these are a tropical fish and need stable temps from 72-78 degrees and I wouldn't recommend anything warmer than this. If housed properly these fish will get huge and be some of the most active and gorgeous fish you've ever seen. Just be aware that'll you'll eventually need to get them larger and larger tanks as they grow, eventually you may need to donate them to your local aquarium where they'll have enough space to thrive or return them to south america and the mangrove inlets where they live and spawn.
Sadly when I purchased these fish I (like almost all other people that buy them) was completely unaware of this despite researching them initially. Anyone unable or unwilling to provide them with the proper and long term care (plus huge tank and brackish water) should NOT purchase these fish. The two pictured here lived happy and peaceful lives in my 40 gallon breeder tank I bought for them up until my poor quality heater developed a crack and promptly electrocuted them. I must say though they were some of my favorite fish ever and by far some of my most rewarding next to my brackish water puffers who would come and great me and listen to me when I would talk to them.