I don't know who made your diffusers. Here is a chart from Vertex.
From all my research, only Vertex publishes these gpm lifting rates. My EasyPro diffusers "look" similar (9" EPDM disks), so I used the Vertex data in my calculations.
Is there any issues to watch out for when running just one diffuser with the 3/4 hp pump?
Yes, there is a potential problem. A single diffuser must be able to handle the air flow rate produced by the compressor. If it can't, the compressor may be damaged. Check the spec sheets of the EasyPro compressors and diffusers. That's why I purchased the 1/4 HP compressor and nothing larger for my application.
I have adjusted the valves so all the diffusers look like the are producing about the same about of air. If I don’t do this the deepest one doesn’t put out much air. The pressure gauge at the pump read about 10 psi. Is this normal and should I be trying to balance out the diffusers output using the valves?
I try to balance the flow rates also. There should be about 1 psi per 2 foot of water depth. So, 10 psi sounds about right for your diffuser in 18 feet of water.
