And more bad news from me after the maintenance concerns. Sorry in advance.
Firstly, how much sand do you have. Is it at least 4 inches deep? If not, its not doing all that a deep sand bed should for filtration.
Change that. Firstly, like has already been stated water change. And lots of them until you water is under control. The PH
and nitrate should be taken care of just from that. But your maintenance has really got to improve for the future. Premix your water and adjust it's PH
to around 8.2 - 8.4. As you change water out your tank will slowly adjust down, same with the nitrates. I'd suggest more than 20% to get it under control. However I would be afraid that with the water the way it is currently, the shock would be very much bad. So small steps for now.
Secondly. When you say you clean the skimmer a couple times a week. Do you "clean" the skimmer or just dump the cup? I don't currently use a skimmer, but from my understanding it takes a while for it to actually produce waste after every cleaning. If you're not dumping a lot of gunk out (especially at your 'trate lvls), you're probably cleaning it too often for it to do what its supposed to.
Thirdly. What is the output of your pump, and do you have any additional flow? You want to turn your tank over at least 10 times an hour, and preferably around 15 times. If you're getting poor circulation, the water won't get to where the bacteria live for the rock and sand to actually work as a filter.
The good news is that your corals will do much better once your water is in check. Your fish will be happier and live longer also.
Now for the bad.
Firstly, how many chromis do you have? I'm guessing when you say "some" to mean at least 3. That means you've got at least 9 fish already and are still adding. A 55gal standard would be very overstocked at this point. A 55 hex, or any tall tank is seriously overstocked. I'll get to why in a minute. Also, tangs should not be kept in less than a 70gal. Even when they're small they need a bigger tank to be happy. They require a lot of swimming room, which you can't get in a hex shaped tank. The bio-load you currently have is going to make having good water increadibly difficult if not impossible. The "inches of fish per gallon" rule does not apply to saltwater tanks. They should only be stocked to a fraction of what the same size freshwater tank can be. On the plus side, any cleanup crew or corals etc. aren't counted towards the bio-load.
Natural filtration is all about surface area. DSB
and live rock have massive amounts of surface area. Significantly more than any WD or other mechanical filter can give. But a hex tank has less available area for the sand and rock to occupy, thus limiting the filters ability to do its job. The water surface area is also important. All the bad gasses have to exit through the surface and be exchaned with oxygen. Again a hex tank has significantly less water surface area for this to happen. If your overflow skims from the surface to pull to the sump, that will greatly aid in the gas exchange. A shallow tank of the same volume gives a lot more surface area for all this to occur in, and stocking guides are aimed at a standard shape tank.
I hate to be a total gloomy gus, but once everything is in check and in control you'll have a nice healthy tank and happy fish. It will be a good chunk of work, but once its done you'll be pleased with the results.