Well pacheco, just how long you had those two fish? You need with those butterfly fishes the highest of water quality and please make no mistake, im not talking about just your water testing for it goes much more beyond that. Like i like to know your filtration method that is on this tank and is it the 4' long 120 or the 5' and how many power heads you have on the inside of the tank for your water currents and as well do you have at least some 150 to 200 lbs of L/R? an d as well what type of gravel bed you have and how deep it is? For your problem at this time can be anything, for one thing your tank hasn't age that much for a three month old tank because for a coral beauty angle one needs a well established tank as well with butterfly fishes and as well they natural diet which is greens for you been placing in the tank dry seaweed/algae once a day for them and what other foods if any you been offering them?
But what i not understand is how can your nitrates be that high if you test it correctly? For how many other fish tank mates you have in the tank? And when you feed, you not just dump a load of food into your tank but small amounts and it might be best for you too shut down your main pump(s) while the feeding is going on but not the power heads on the inside of the tank and not forget to turn then back on after the feeding.
Now a Ph of 8.0 is OK, but i would raise it a bit to as high as 8.2 or 8.3 for you see you are at the border line for any number of marine animals when the Ph drops to 7.8, they not would have an appetite and would be feeling stress. For like most all eels will give people this type of reaction to low Ph in their tanks.
But from the sound of it you do a weekly 20 gal
water change, this is obviously not enough of a water change and as well be depending on your bio load for that too causes you too over feed in which case i would say for now to do a weekly water change of 30 gals.
The other thing is both those fish that died live among the reefs which the pH is a bit higher as well as the salinity but it not means they should do as well with lower amounts as you have, just need the highest of water quality with both. And I see you add only dead rock is it, this might be OK if you enough L/R.
OK, after i finished reading your post as i responded to it, i see at the bottom you all those fish in this 120 so i know now that you are over feeding and i will have to say to do a weekly water change of 35 gals or before you know it, your nitrates will be through the roof so i will have to say that stress killed those two fish
And i see you not enough L/R for 70 lbs is little for a 120, least by what i understand, i would double it. But that is just my two cents worth of it. As well as mentioned, you over loaded that tank way too fast for in some just 6-7 weeks you all those fish with 80 PPM
nitrates. So it is surely a no wonder those two died
you need to cut back your bio load right now, if you not, it will only get worst.
and yes all the fish are rather small about 26" in all
A fish like that clown trigger will grow quite fast and will begin to attack many of those fish you have there and you should never just look at it as it is now, that is always the mistake most start off with and as mentioned, we all hope you learned something here and please do return many those fish before a higher death count begins and allow your tank too age for you must be having nitrites an d some ammonia, you got to have it with a newly cycled tank as over stocked and over feeding.
And do you have a skimmers? which model?