Thanks but what should I do about it? I don't see any anywhere else in the tank. Should I go through the hassle of trying to siphon it out or just leave it be?
I didn't do it tbh. It will be a real pain since it's low to the ground and I would have to do an old fashion gravity siphon. Also considering the toothbrush and net method. But may just end up leaving it.
Microalgae in the refugium is normal. I'd assume your return pump is sharing the same chamber as your refugium that is given the big clump of chaeto on it. Just think about what would happen to the glass on your mt if it were left for months without cleaning. Imo you can leave it and it won't harm anything but if you want it gone manual removal is the way.
As for phosphate what are you testing with and what foods do you use?
Hanna and it's pegged 0, dang. You have a good mix of food so it doesn't seem like a low phosphate ratio so I would consider dosing phosphate directly. Otherwise you could drop the mysis/brine and go all in on high phosphate food but I personally see no change in phosphate from changing food. It is confounding because you would at least think it would affect the dosing rate of sodium phosphate and yet I am steady at .01ppm/day no matter what I do.