I think Wamp gave very good advice.
Tap water is likely to contain nitrate, phosphate, and other bad things. If there is nitrate, your water changes will not be very effective. RO/DI water is the best way to go because you are adding zero nitrate, etc. If you can part with a fish or two by taking them to the LFS for credit, it would be a lot easier to get the tank under control. You can do water changes as often as you want if (a big IF), the water temperature, salinity, and pH are a good match for what is in the tank. If you get careless and put too cold of water in, for example, you are likely to see a major ich outbreak. In a crowded tank where the fish are stressed, diseases are usually fatal and can kill all of the occupants.
Feeding that group is a challenge. As mentioned earlier, they are messy eaters. You put a chunk of algae on your clip and they will eat some of it and break a great deal more loose. That loose material will probably gather on the bottom and deteriorate. Why should they bother to pick it off the bottom when there is more on the clip? I would suggest leaving the algae on the clip for only a couple minutes. As soon as no one is actively picking at it, get it out of there. Let the fish patrol the tank picking at live growing algae or scavenging any that was broken loose from the clip. Feed very small amounts more often so nothing reaches the bottom. This is a lot easier said than done, however, because your fish will compete for the food. Your problem will be getting enough to the least aggresive fish.
The nitrate level of 80 by itself is not necessarily dangerous for those fish if it does not change dramatically. Most fish can tolerate levels that no coral would survive. That does not make it a good idea though to maintain those levels because the fish will be stressed to some extent. Also high nitrate will encourage all sorts of undesirable algae growth over time.
A tank full of large attactive fish can be so beautiful, I understand how you got into your predicament. Maintaining such a tank is extremely difficult. I wish you luck.