DI is a must have. Ro will bring your tds about 95% down. That 5% makes a huge difference to having nussiance algae problem. Also get a chlorimane blocker with the carbon filter you going to replace
Nothing you can do about the hosting. Might try different corals. My baby lightning is hosting my hammer lol because the bigger clowns won't let him get a nem. They guarding all 4 nems from him
The blackspots are call bullet holes. That is a design for that clown. The more bullet holes the higher the grade. Hosting takes time. Not all will host something, but some will host anything