Reading this thread over I don't see any details of the setup as far as Live Rock and sand -vs- Dry Rock and sand used to start. IMO/IME if it were Dry/Dead then 4 months is very young for that bio-load. LR is/can be years to decades or more old and fully colonized by an assortment of nitrifying bacteria. I cut open a piece of Marco rock from a friends tank that was 3+ years old and barely populated more than 1/4" - 3/8" in by bacteria or in essence as effective as maybe 15% of LR of same volume. Also look into seeing if you can get a copy of your Water Districts water analysis, we in Washington have some of the very best aquifers in the country though there are some nasty ones as well full of heavy metals. I imported and bred Discus in the mid 70's til the late 80's and chose location of my home for best overall Water District around, slightly soft and acidic and non-chlorinated which was perfect for my 200+ gallons of WC's daily. About 15 miles North was/is an aquifer that you cannot keep SA Chiclids, Catfish or scaleless fish alive without major adjustment because of heavy metals (just below human safety concern levels) Copper, Selenium and Cadmium in abundance. Just my $0.02 and would be worth investigation to see whats really in your water. Obviously keeping Discus there shouldn't be an excess of heavy metals but something else maybe ???
Cheers, Todd