Your Salinity was fine, the person(s) who told you other wise are not knowledgeable. What was the method of testing your salinity? Those cheepie plastic hydrometers are not very accurate. If this type of hydrometer is what is being used to test your water, I would suggest that you use only one hydrometer to measure your waters S.G. If your hydrometer at home is inaccurate, it will be consistent in it's inaccuracy which, assuming that it is close to accurate, is better to maintain it at that constant value versus adjusting it because some one else's hydrometer measures the S.G differently. May be a thought to either buy a refracometer or at least have your hydrometer calibrated against a refractometer.
If your aquarium genuinely had a ph of 7.8, what contributed to it dropping? If the RO water you added to dilute your aquariums salinity was good quality RO water, it would not have raised your PH. If it did in fact raise your PH, it was not quality water. Have you tested this RO water or are you guessing it is good quality? What is the TDS of this RO water? Are you certain the results of these ph tests were accurate? What brand test kit was used? How qualified was the person(s) who performed the test? From your description, either the tests performed were inaccurate or that was bad RO water. What about your waters other parameters? Because your considering using super buffer, what is your alk? This is very important to have determined prior to this products use. You easily can cause much more problems than solve problems if mis application is done. If your ALK is fine and your adding Super buffer, you can raise your alk to high very easily. This product and products like it must be used very carefully, if you add it full strength like the instructions suggest, it will almost instantly raise your alk & ph. This is very stressful to just about everything living in your aquarium. Adding very small doses over several days is a more desirable method. If your ph was legitimately 7.8 and you have already raised it to 8.1, you have already cause a bunch of stress to your aquariums inhabitants, if additional adjustment is needed, take it very slow. Additionally, back to my question of what contributed to your ph droping, if you have not determined what dropped it and corrected it, the adding of a buffer my be temperary. In other words if you have not addressed / removed what caused the problem in the first place, the problem will still be present in your water, the buffers affect will short lived. This is even more harmful to your aquarium than just leaving the ph at 7.8.
If you don't have a test kit, you really need a good quality one. If you do have one, what brand & why have the inexperienced people at the local chain stores test it for you? A test kit should not be considered a luxury item, it is a must have piece of equipment when owning a salt water aquarium. You need to test regularly so you can learn what your aquarium does and so you can determine if what maintenance your performing is adequate.
To really be able to offer responsible advise, answer the following question. This will give a better picture of what is going on and make it easier to give some ideas on what to do.
What size aquarium?
How long has it been set up?
What type of filtration system do you have?
What type of maintenance do you perform and how often?
What live stock do you maintain in your aquarium?
Specifically, what are your waters parameters;
S.G.
PH
ALK
Ammonia
Nitrite
Nitrate
Phosphate
(If Keeping live rock & Corals?) Calcium & Mag
How long and constant have these values been maintained and stable?
I don't see anything that stands out that would have killed your yellow tang. May just have been a poor quality fish. Did you, or who you got it from, perform a quarantine process before putting it into your aquarium? Was it eating and if so, what? Any sign of disease or parasites? How long did you have it? What was the quality of the enviroment it came from? I don't know that I can offer any advise as to why your tang died but with the above questions answered, I'm for sure that you can get some answers that will keep you waters parameters more stable which in turn will increase the success of your tanks inhabitants.
Joel