dont get the tang. scrubbing the rocks and manuel removal of as much as you can goes a long way though. did you start this tank with tap water?
you can just throw a blanket over the tank so little light gets in or at least keep your lights off. good flow via PH's and little to no feeding. do daily blowing off of the rocks and 5gal water changes with RO/DI water. run some blackdiamond or seachems carbon to deal with the algae die off and also run some phosban or rowaphos to remove phosphates. get some buffer and keep the ph around 8.3. keep temps good. do this for about a month. after a couple weeks you can decrease water changes but i would still do at least 2 a week till all looks clean no phos, NH4, NO2 or NO3 present on testing, or very low levels, then start adding light 1 hour aday till your back to your normal photoperiod. once things are back to normal make sure you get a good cleanning crew before the algae returns. i suggest adding something like:
10-cerith snials
10-nerite snails
10-trochus snails
1-nassarius snail, 2 max.
if the algae is not that bad you can just add the snails in a couple groups a couple weeks apart and reduce your lighting period, decrease feedings, increase water changes using RO/DI water, add the carbon and phosban/rowaphos.
the BIGGEST suggestion i have is get a good
skimmer and of course one could always use a bigger tank, to fit fishes like that tang.
