Four Yellow Tangs Dead

Yellow tangs need a bigger tank that a 30. In smaller tanks, they are very stressed. My honest recommendation is to find something else, that will be suitable for the size tank you have going. Sorry. With the live rock, you could try a lemon peel angel--adult size is around 4 inches, same bright yellow with blue rings around their eyes. Much more likely to live in a 30.

your Ph is a lil low and what size tank did you have these tangs in? Yellow tangs need atleast a 90g tank!

I would agree ph should be in the 8.3 range. Tank sounds small for all the livestock you have.

the wrasse will also need more room!

Zebrasoma Flavescens. Agree this species should have more room, however IMO unlikely that the tight quarters would kill four of them in 6 months. I suspect you are either not feeding correctly or you are getting poor quality fish from the same supply chain.
Years ago when I was in my late teens, we used a product called Tang Diet; a silly-looking frozen product that surprisingly had no visible greens, more like oatmeal cake. At any rate, with only limited experience, I was able to easily maintain yellow tangs on this food for years; even in smaller tanks. Today, some are fed nothing but flake food and terrestrial greens which can create serious problems. Try dried and fresh algae products for fish/human market, soaked in Zoe/Selcon, what have you. Fenner notes that some sources have higher mortality rates/poor handling and shipping issues; as a former collector and retailer I can tell you many marines die within 30-60 days because of poor capture/handling techniques; in such cases the fish may feed and appear to adapt well, but are doomed and I don't mean from cyanide; other issues at work here; some as simple as lag times in feeding and handling.

Small tank, already with 4 smallish fish. Hair algae, low pH? Sounds like a water quality issue to me - just because nitrate is at zero means nothing, the algae is chewing it all up, leaving nothing to test for.

If you want a yellow tang I think you'll need a larger tank and get your water sorted out. More water changes, better protein skimmer

i'm pretty sure distilled water takes out essential nutrients and vitamin and stuff. I dunno I may be wrong but thats what i've heard.

Distilled water doesn't contain much in the way of dissolved minerals, but the slat mix replenishes them. Distilled shouldn't be used straight in freshwater tanks, and used with caution in SW setups--some distillation processes can increase the copper in the water, resulting in problems for invertebrates.