Hi, there. I have 2 filters running on my 55 gallon tank - one is an Aqua Clear 300, and I recently added a used TetraTec 300 that I found on Craigslist. I know they don't make them anymore, so I'm having trouble finding any information on them. I've had it running for a few days, and it looks like a great filter. Seems very powerful, and it moves a lot of water. What I don't understand is how to adjust the intake/output. If anyone has one of these filters, hopefully you will know what I'm talking about. The problem I'm having is that I don't know which direction to turn the big knob in to adjust it to full intake. I can't tell when it's on full intake, because if I have it on full output it drains out very quickly and makes a very loud gurgling noise while it fills back up. There's the normal spout on the front, then there are two holes on the bottom that drain the bio chamber. I tried turning the knob on the inside to shut them, and I tried plugging the one that's covered by the clear globe with a sponge, but then the media chamber overflowed. I guess what I'm saying is that there's something about the way I have it set that makes it so that the intake can't catch up with the output, but since it's old I can't see any of the arrows or marks, so I don't know how to turn anything . I have played around with it for literally hours now, and no matter what I try I can get it so it's taking in and spouting out a bunch of water, but after about two minutes it gurgles and gurgles and has to take in more water before enough spills in to spout anything out. I know this question is very complicated, but if you have one of these filters I'm sure you'll know where I'm coming from! Thank you so much.