Those SS65's are junk IMO
. I wouldn't run one of them on a 30 gallon.
I would definitely invest in a much better skimmer.
If you are considering adding a sump fed by a HOB
overflow (I saw that you couldn't drill the tank, so this is the only way besides HOB
equipment) You must first decide where you are putting this sump.
If it's inside the stand, a 20 long is probably the largest tank you'll be able to fit. Buy the 20 long, and take it to a glass shop and tell them to make 3 partitions for it, and they only need to sand one edge (the other 3 will be glued in and will not be a hazard).
All three pieces are to be the same size, and you probably want them to be about 8"-10" tall. You will figure out the optimal height when choosing your skimmer and finding out it's water depth requirement.
Measure the inside of the tank and if you can fit it (inside dimension would have to be more than 11.42") , I would get this skimmer-
Bubble Magus BM 220A Protein Skimmer Brand New | eBay
If you cannot, then I would get this one-
Bubble Magus BM 180A Protein Skimmer Brand New | eBay
Of course you would need some aquarium silicone. First section will be intake- skimmer and maybe room for a filter sock.
Second section would be a refugium, and the last section would be where your return pump will go.
The last section must be large enough to fit your return pump. A mag 7, 9.5 or Eheim 1262 should be fine pumps for a 75 gallon tank. You will have to get an overflow rated for a little less than the pump you choose, or vice versa.