I've got several Tidal 75 HOB filters... taking the plunge at modifications...

my biggest complaint with these filters, is that infernal sucking noise... that I always thought was the protein skimmer... come to find out there are 2 slots, cut into the body of the filter in the waterfall return, SeaChem calls the "aerating return flow" it drives me crazy, if my tank is full, it’s reasonably quiet, but if the water level drops down about a 1/4 of an inch, I get this horrendous sucking noise... the skimmer slots could do the same thing, but the noise that bothers me most are the 2 slots cut in the discharge, the 2 slots to the left in the picture are open, the others are closed… those will be covered with silicone, & while I'm at it, I'll also silicone 1/2 to 3/4 of an inch of the very top of the 3 inch skimmer slots... next, there is a dial under the filter box, that the draw tube is half blocked, & you can adjust it to completely block the draw tube, so it only sucks from the slots at the bottom of the pump box, which is 6-7 inches from the top of the tank, and the skimmer slots at the top... I have no need for the pump, not sucking from the bottom ( perhaps it's there to keep from sucking up sand with earth eaters in the tank, or to provide more suction for floating big pleco poops??? ) my tanks that have sand in are tall tanks, so not a concern at this point... but my next tank will be a normal height sand tank, so I'll revisit this mod, on my next filters... I had issues with one pump squirting water out the top of the box, when the filter sponge got too plugged, will look at that later... I think a tighter fitting sponge, or thin sponge inserted along the box wall, will fix that... I think these are one of the best HOB filters out there, if it weren't for those 2 slots in the discharge… I wonder if it was a mid production filter improvement, as most of mine don’t make the same noise… the pictures included here, are on a newly purchased unit???

because I didn't want the big hole in the center of the bottom last picture above, I put the baffle & dial back on, but turned it so it's on the other side of the draw tube & slightly restricts the slots in the bottom of the filter box, rather than the draw tube

Here is the one, making that horrible sucking noise water level is down 1/2 inch from as full as I can fill it… would guess highly oxygenated, which is what I’m after in this tank, but unwilling to put up with the noise

Not much silicone needed… just in the air intake slots, and the very top of the skimmer, I mostly wanted to cover up the slot parts, that pointed straight up

Wow! These filters are complicated.

So I'm really happy with the 1st filter I modified, whisper quiet, even when the tank level is 3 inches down ( mid water change... it would never normally go that low ) in the 1st installment, I didn't get a picture of the factory draw tube baffle... in factory condition, you could completely block off the draw tube with the adjustment dial, and the factory baffle ( something most people would never do, & could easily be accidentally blocked as there is not an open / closed indicator ) since I don't remove the factory adjustment for the tube, I could reduce, but not completely block the tube, after the baffle is removed... this picture shows the pump housing box, that just pulls off, with the adjustment in place, but spun around to not block the tube ( my use position, and can easily be returned to the factory position ) and showing the factory draw tube restriction baffle, that I removed in the 1st installment, & will be repeating on my 2nd filter modification...

Pulled the old filter body, from my African tetra tank, and insert the rinsed old sponge ( along with a new replacement also ), and rinsed bio media, back into a filter cartridge, and put it into the last filter I modified… I would guess the flow increased 25%, even though now there are 2 of the coarse sponges… I’m liking this modification, much better than the factory set up… will be starting on box #3 right away…