your water is fine, and keep up with the water changes...try 25-30 percent twice a week...I do 90percent twice a week and when i do the water changes there is about enough water for them to breathe as they are on their sides swimming. so i leave about 10 percent of the old water. my trick is not to clean the filters or gravel as oftne as i do WC. and when i do clean the filters i do not do a WC the same time. i do them on seperate days so that i dont loose all my bio. and when you clean your filters use the tank water...
as for the stableness....well its a victorias secret kinda thingy...gotta keep trying combos of lighting, substrate, and background or even tank mates. my tank is bare bottom but i have plenty of hiding places. no background and "LOW" lighting. mine are always stable as in the NTT and IT. it could be that you have a black background so they are trying to blend in and why they are always dark or unstable. or it could be that you need some more hiding places for them as i have found out that dats dont like to swim in the open. they like their privacy and teritory...
as for food, if you want to make them grow fast, just do feeders and frozen foods. i feed mine 3 times a day for the first month i got them and now i feed once a day at night when its dark and they come out. i find that if you feed once a day at the same time, they tend to all come out as they know this is the only time food comes and if they dont eat they other fish will. i feed mine, granules, flakes, pellets, frozen bloodworms/brine/mysis shrimp, market shrimp, smelt and feeders once in a while. but my feeders are quarantined and medicated and fatten up with vitamins and good food for 3 months cycle before i feed them to my fish. so if you go feeders, just be carefull...
IMO, i think you need a pleco for the uneaten food, and dither fish as in angels(not aggresive fish, but when it comes to eating they make other fish eat!). some driftwood and more hiding places.
you didnt say what your PH was and that might help...as for them scratching, its a sign of ICK. they are scale less and prone to ICk also. keep an eye on it and do what you did, salt and turn the heat up.
sorry i can give you more info right at the moment, but if you have any questions, feel free to ask. I've kept dats for over 15 yrs. i'm still no expert but i can share my experience with you and hope that it helps.
GL and welcome to DAT keeping!
BTW if you decide to let some of the dats go, i could use some more NTT's. I have other dats for trade....