I don't think 6% per week is going to help much.How often can you commit to a water change??After you figure?that out, then?find a balance between bio load and maintenance. ?IMO, you need to address at least 3 things to focus on the excess nutrients/algae (not replenishing trace elements needed for the corals)?and help get to more time?between water changes, CUC, ?bio-load, and export.?1.CUC?- Others can chime in here but for 75g with a sand bed IMO: 4?Brittle Star's or?Serpent Star's, 2 x Emerald crabs, ?30 x Nassarius Snail's, 50 x?Banded Trochus Snail's(I don't like Astraea's) ?, 30 x ?Dwarf Blue Leg Hermits(I don't like red's), and the family likes to watch Blue Tuxedo Urchin's(not me). Maybe 20/40 on the snails, but you should assume die off.2. Bio-load?- IMO, Keeping 3 healthy feed tangs in 75g while keeping nitrates low enough for healthy coral with less than monthly 20% WC is near impossible. I would guess replacing the 3x tangs with smaller fish (6 line wrasse, royal gamma, a pair of?Banggai Cardinal's) would [drastically] reduce your nitrates. Smaller fish = less food and poop. ?Something to consider, I ?had a single clown in a 46g sps tank for a year with no WC. No, the SPS where not thriving, but I would give them a 7 out of 10. ?3.Export -? Reduce Nitrate and Phosphate to help keep the water clean between WC.?Without almost daily water testing testing, organic carbon is out of the question. Without [at least] bi-weekly changing carbon and GFO are out of the question. Sticking with the basics, What else is in the sump? ?Does your sump have a fuge and are you growing some?macro algae like?chaetomorpha? Grow some chaeto and pull some out as needed. I remove at least a gallon weekly with my?skimmer cup cleaning. Also,?load your sump?up with surface area for nitrifying bacteria?with live rock and or Marine Pure. This also provides a place for pods to grow that help reduce the fish food needed. That single clown had a feeding schedule that was also terrible.If your not going to change them regularly, ditch the filter socks. It does help to remove solid matter before it breaks down. The good news you can have that with lower maintenance intervals, check out the Automatic Roller Mat by?Theiling. The bad news, money and space needed.Finally, I forgot something I should have asked about first. I hope you are only using RO/DI for top off.Even with a sustainable bio load/export program for Nitrate and Phosphate?, eventually the elements beneficial to the corals are going to become depleted. If the WC schedule does not keep up with this, the corals would require a dosing program to keep those levels acceptable. I.e. my 1 year WC SPS tank was topped off with RO/DI kalk and?mag was high for?bryopsis.
