here are few very likely causes.
1. too many fish too fast.
2. there are still ammonia readings, which is extremely stressful on fish. and stress causes ich.
3. nitrates are WAY too high.
4. MOST fish arrive stressed, due to being captured and shipped to an lfs, then taken from the lfs and put in your tank. that's stressful. imagine being snatched off the street, and stuffed in a box, only to wake up in a prison in a foreign country. same thing.
5. i would highly recommend you return your goby and mandarin. they need highly established sandbeds, in order to feed well. a 30 gallon tank is generally too small of a tnak to adequately support a mandarin. though there are always exceptions to every rule. i'm not saying you HAVE to, but it would be in the fish's and your best interest.
6. do not add any more fish to the tank AT ALL until all your paremeters are good. NO ammonia, NO nitrites, and preferably less than 20ppm on nitrates.
i would raise the temp to over 80, i KEEP my tank at 82. 82-84 would be a good starting point imo.
i would also mix in some fresh diced garlic with thier food. this boosts thier immune system, and also makes them "less tasty" to the parasite. sort of like the garlic pills for dogs/cats, for flea control.
i would NOT use copepr treatment if you even THINK that in the futyre you MIGHT consider a reef tank. if you do, then you'll have to buy all new rock, pumps, tank, sand, etc.. that tank will never support reef etc.. and its very likely, that adding copper treatments will kill your hermit crabs, and star.
you should do some water changes to get your nitrates down, that's a really high number, not only does it couse stress, but extremly high nitrate levels can also burn the fishes fins right off.
keep in mind too, this is just one persons opinion, i woudl wait for more feedback before doing anything. i can opnly relay what has/does work for me. but there are lots of adequate ways to accomplish the same goal.
hope it turns out well, keep us informed!
jay