Although there is a lot of debate on the subject; IMO & IME, RO/DI water is not always a requirement for a healthy SW tank, especially a fish-only tank. Much depends on your local water supply. You can get a report from your water company that will give you some info and you can test your water for nitrate, phosphates, silicates, whatever--then decide. I've kept fish for a long time, in 3 locations, without it. I know there many successful hobbiests on this forum who don't use it either. I would never advise anyone not to use RO/DI; nor would I say it is always necessary, because it isn't.
As to the bi-color. This was once considered an "expert only" fish and has now become a bit easier. I'm sure cyanide use and proper handling are the reasons for so many failures. In any case, I don't think it is a fish for a beginner. I also assume you've found out that pipefish are very difficult and usually need their own tank.
On the feeding: IMO, just adding garlic or changing food is not going to save a fish behaving like this. A healthy fish will almost always eat an appropriate food, it may take a while though.