Thinking that it is new tank syndrome. Although the tanks have been established since December, you've been adding bateria to a tank that has been estalished with a bac. quantity to support the fish. Adding more bac, to the tank is like trying to start a fire that is already lit. Considering the size of your tanks, the water quality changes quickly.
I'd would do a 20% water change in each tank now and continue to let things settle.
Do another 20% change 2-3 days after and then again as needed...but don't jack with the gravel but once a week until things cool down.
Only vacuum a 1/3 of the gravel, each week but don't dive the cleaner into the gravel, just slightly brush it and vac the debris.
My thought is that you've got a crazy amount of alive and dead bacteria looking for a home and there's just no place to go. Stay the course with the water changes and give it time....although that can be the hardest thing to do! I'm there too!
