Been keeping fish for 7 years , I have a 55,125 and now a 75, fully aware of the cycle, I have had other tanks as well, I have cycled fish in and fish out, I always like to do fish less cycles using stability and always been successful doing so, always use live plants and prime, I do weekly water changes with my python , baffled with this 75 gallon tank , bought it used , got it home rinsed it out then cleaned with bleach as the lady said she wanted to sell it as her fish kept dying, I took this as she prob didn’t know what she was doing like not cycling the tank or water changes etc, so get the tank cleaned with bleach/water mix , rinsed thoroughly, filled with water test for leaks then drained it, built stand for it set it up, I have an ac 110 running it heater keeping at a constant 82 which is in range for barbs, planned to make it a tiger barb (species only) tank, gravel, live plants, driftwood, and rocks , lots hiding spots for them , ran the tank for three weeks dosing with stability to the T per the instructions , tank wouldn’t cycle, I have never had a tank not cycle after three weeks,I have cycled so many tanks, so figured well let’s put some fish in kick start this bad boy got 15 tiger barbs after a week no ammonia ….at all, I put one of my hob filters from my long time established tank in there for a week , i tested daily never saw ammonia go up, no nitrites , so now this thing been running for a month ammonia 0 nitrites 0 but nitrates are 5.0 well I figured maybe it cycled thanks to my established filter but didn’t notice cause nitrates are the last thing to go up, the ammonia first nitrites second then nitrates right? I never fully believed this insta cycle using established filter but maybe ? I don’t know, but my tiger barbs are kickin off left and right , no signs of disease , got plenty oxygen using power head facing surface an obviously the ac110 on the other side breaking the surface there, I feed twice a day quality omega one fresh water flakes and blood worms, gravel was new, filter was new , rocks were in an established tank with two healthy oscars and plants were from another established tank, what in gods green earth would be killing these guys off?! I thought maybe didn’t get all the bleach out? but couldn’t smell any and now it’s been a month , started water changes after added the fish three weeks Of running , anyone ever experience this? Did I add too many barbs at once ? Fish had no ich, fins are not clamped , no hole in head, there laying on there sides at bottom breathing fast or floating on side at top breathing fast , all the other bigger ones are fine not going to surface I know I got plenty gas air exchange ( oxygen) in the tank, sorry for the long post but I cannot figure this out and and lost a lot of money and fish over this , I am by no means a noob but we all have room to learn and improve , Amy advice or help would be greatly appreciated thanks.