I bought a used euro reef 135 that i placed in my sump at 7 in and when i open the standpipe and the air valve all the way open, the pump rattles alot. If i close off the air little by little it will stop. What is going on here?? And also i have alot of bubbles coming from the exit tube of the skimmer. Is this normal? Also is there any info on what the bubbles should sit until the skimmer starts to skim??