I have the same problem and I can't move the water level because my skimmers in the sump. What I did was add live rock where the bio-balls were and that cut down on the splashing noise...that reminds me I need to buy more. I removed the bio-balls very slow and have been adding live rock in the sump the came way slow. Hope that helps if you skimmer is in the sump
I guess I should say that if you have gurgling sounds you will want to add a vent. Some systems don't have it for various reasons and other do. I needed to vent both my drain for my over flows and my drain for my skimmer.