I'm looking for a air pump that can handle at least 750 gph. Please Pm me.
If you bleed off the excess air it won't hurt the pump. Its only damaging to the pump if 'back pressure' exists. There are commercial bleed valves that can be purchased, or you can make something simple with a ball valve. Noise level will depend on how much air you're bleeding, but some of the commercial setups have diffusers to help with the noise. I think Christine had some pictures of her setup that helped control the pressure and the noise.If I got a bigger pump for future tanks, would it be damaged running only on the few.
A separate pump for those tubs is going to be easier. The deepest tank I have is 24".The other consideration is how deep do you want the air to bubble in the tanks. That's always been my limiting factor. Having a slightly larger pump will cover you in the case of a deep tank or a long run of airline tubing.
BTW, how are you planning on getting the air out to your outdoor tubs? Are you thinking of using the same pump as inside and running a long tube out a door or window to the tubs? If it were me, I might get a smaller airpump for out there if there's an outlet available.