Wow. That tank needs some serious help. As mentioned, it might be past just adding a CUC. I would scrape the sides, remove 1 rock at a time and scrub Each one, but just do 1 rock every week or so. It didn't get in this condition overnight, and it won't be an overnight fix. Then A good sized pwc
is in order and try to suction some of the stuff off of the sand. What is in the hob
filter? if it has the wheel, then remove that wheel. The hob
filter is fine to use, but needs a cleaning regularly. I have a hob
filter on my 30g with a sponge and chemipure in it. You could put a turbo snail in there. Two clown fish really are too much for a 10g. Good luck!