It appears as though you had too many fish too soon and the wrong fish as well for the size tank. The only thing you have left that's ok in that size tank is the shrimp and what you had in there that's ok was the clown. There's a lot of stress going on or was going on. The population has dwindled to what it can handle. You didn't mention the ammonia levels and that can change that last sentence.
You have good filtration and if you have live rock, it's even better. A 25 gallon tank for marine animals is small so you need to keep the population down and the limit the species of fish that can be housed. Stick with fish that reach no more than 5 inches adult size.
At this point in time, you need to make a decision about the remaining occupants. Both snappers and tangs get WAY too big for a 25 gallon. Bringing them to a LFS
would be the best thing to do. Do not get any more fish or inverts for at least a week or two weeks and tend to the tank.
Nitrate levels are not bad, but should come down, especially if this tank is going to hold coral. Time and the protein skimmer will help you with that. Do a 5% water change every other day for a week. Then maintain a routine of 10 to 15% once every two weeks. Don't worry about feeding the shrimp much. Offer a tiny bit of flake once or twice a week until you are situated with the tank again.
In the mean time while waiting for the tank to settle, think about the things you want in there and always keep the adult size in mind when populating the tank.
Do you have live rock?
What are the ammonia levels?
, to treat red slime algae which is actually a cyanobacteria harmful to coral, use ChemiClean or Red Slime Remover and use as directed.