So, my husband and I just got a used 125 gallon tank. We cleaned it out with a half vinegar, half water mixture. We rinsed all the gravel, decorations, and fake plants before addding them. Now, we also have a 54 gallon and a 10 gallon. Both of those are fully cycled and established at this point and both were established with fish as opposed to doing a fishless cycle. We had great results with both, losing very few or no fish the first month. We haven't been as lucky with our 125... We added 2 young Bala Sharks 24 hours after the tank was set up. A few hours later, we added 2 young Albino Tiger Oscars. Everyone seemed to be doing very well until suddenly, we look over and one of our sharks is inverted and caught on a plant, barely breathing. I tried holding him over one of our bubble stones in case he was having trouble that way, but it was clear he was too far gone. I immediately tested the water. pH was a tiny bit high at 7.6. Ammonia and nitrite tested negative and nitrate was maybe about 2.5 ppm (perhaps thanks to the bacteria I added at the start??). About an hour after that, I suddenly see our other shark dart up towards the surface like a bullet, do like a crazy flip, and suddenly it is drifitng upside down as well. About 20 mins later, he was swimming upright and appeared fine so I left him be. Unsurprisingly, I found him dead this morning. Does anyone have any idea what happened? I was hoping Bala Sharks would be a good way to start my aquarium's cycle since they are supposedly so hardy. My 2 Oscars are still in this tank and besides laying around on the bottom for a while this morning, they seem okay... What should I do? Should I keep them in this tank? Or should I pop them back into the 54 gal for a few weeks until my 125 is cycled? I've never done a fishless cycle before, but I don't want to lose my Oscars. Would putting them in the 54 be stressful for them since it technically isn't big enough for them? They're only about 3 inches long right now, but I know they can get to be like 14 inches. Let me know if I need to post any more info!