I posted a bit ago about micro bubble jets occurring randomly. I thought I had it figured out but I didn't. I have some more info. Hopefully someone can figure this out. I have a 44 gallon Planet Aquarium tank with overflow in the tank. The pipe going into my wet dry filter is 1 inch and a .75 return. The wet dry filter below has 3 chambers. The first chamber is where the flow enters. It also contains a HOB skimmer. It then overflows into the front chamber and down, through filter floss and other media. From there it flows into the back chamber, which is the return. Here's what I'm seeing. The overflow box above, in the display, will occasionally drop the water level in the box really low. It gets lower than the u- shaped return pipe. It will then make a loud gurgling noise. When that happens, the return chamber below will fill up to almost overflowing which then overflows through the other chambers. After that, the first chamber will get really low, low enough that it can't flow into the second chamber and into the return. When this happens, the return pumps water out until the pump hits air and that causes the micro bubbles. The first chamber then fills up and overflows and starts that sequence all over again. The strange thing is that it doesn't always do it. Sometimes it will run fine for hours and sometimes it will do the above over and over again. The added issue is that I can't use an ATO because the water level in the sump is never consistent. I have no idea what is causing this. The overflow above looks fine. It has an air hole in the overflow pipe, which I understand is supposed to stop the gurgling sound. The plumbing looks fine. What am I doing wrong? Any advice would be great. If you have questions let me know. I work nights so I may not answer immediately but I will answer when I wake up. Thanks guys! Edited to add: I'm getting a new sump with refugium in the next week or so but I want to figure this out so I don't have the same issue with it.