ejrfan,
How long have the diffusers been in the lake? What type are they? It may be time to clean them if they are a stone-type. You may not be receiving their full potential if they are clogged with calcium deposits.
At 25' of depth, do you have one of them at that depth? If so, your compressor may be working too hard to get the air to that one and not allowing the correct volume of air to the other two.
Just some thoughts on how to tweak what you already have.
