What did you put in to get your GH to go from 3 from the tap to 7 in the tank?
If you have large amounts of gunk rotting somewhere (in your substrate, under your drift wood, in your filter, etc) or lots of fish, you will be creating lots of nitric acid that will drop your pH and KH.
My tank has a similar GH, but the pH would be higher than 8 (closer to 9) if I used it straight from the tap (it's well water). I keep Discus, so take it down to closer to 7 with muriatic acid before I use it. In this range, the pH drops about 0.2 - 0.3 pH units per day because of the nitric acid build up from the breakdown of the fish ammonia and feces. Discus eat lots of stuff! This is a 55 gallon tank, so it should be slower in a larger tank. Because it's discus, I change the water every 2-4 days, to keep the nitrates at 10 ppm or below, so I don't worry much about the pH drop. I just add about 1/4 tsp of baking soda a day, to keep the pH at the same place each day. I also don't worry about sodium or tds because I change 60% of the water every 2-4 days.
The crushed coral idea should work for you, if everything else is OK. The complication of adding coral to pH 5 is that you will get a bunch of C02 liberated in the process. Could be a problem for your fish. If you do start adding it, do it a bit at a time. Or better yet, do a bunch of water changes to get your pH to at least 6.5 or so before you add the coral.