OK, I wasn't sure but I thought that might be how you have it set up. What you have is a recirculating water culture system. I can see two simple ways to fix your problem. The first is to change your T connection around like I show in the first attachment. That will equalize the pressure to both sides. Try and keep the length of tubing to both sides/tubs close to the same length. They don't need to be exactly the same, but you don't want one side 8 inches long and the other side 3 feet long.
The other option that I show in the second attachment is to simply bypass the middle tub altogether. If you
pump from the reservoir all the way to tub 2 bypassing tub 1, all the water still needs to pass through tub 1 to get back to the reservoir. Making one continuous circulating loop.
Either way there may be the possibility of the tub's overflowing if the return tubes (32mm piping) cant handle the amount of water volume your pump is putting out. I realize your return piping is almost twice as big, and that's great because it needs to be. Even if you haven't experienced it backing up so far, once you change it, you may get more water volume from the pump than you did before. So you just want to keep an eye on it for an hour or two, and if it doesn't start backing up by then, it wont unless your return lines somehow get clogged with debris.
If it does start backing up just reduce the water volume from the pump. Either by a adjusting a valve on the pump if it has one. Or install a ball valve in the water feed line from the pump to the tub's and close it off some.