Can I ask what a "flow through" is??? Is it like a wet/dry system??? Really high ph isn't good for discus like a salt water tank. You want to keep it around 7.3-7.8 and we usually don't test for calcium. Ph, nitrates, nitrites, hardness and ammonia are the main things we test for. Water changes are a must or you will get sickness or really ugly looking discus. Every day water changes of at lest 50% is great but every other day water changes of 80% are good to. They are a lot harder then salt water IMO because they get sick so easy. Another good thing for discus is a good filter or filters. I have a 125gal and I run a 125 wet/dry system, a magnum 305 with bio-wheels, XP3, fluval 404 and some times I think thats not enough. You don't need a 125gal but I wouldn't get any thing less then a 40gal and even that you might have to upgrade when they get bigger. If you get small discus you would want to feed at least 6x a day they eat a lot and they do better and look better when fed at that amount, but if you can't then no less then 3x a day. Here are a couple of other forum besides DAAH that are really great and have some awesome people there. Hope this helped.
SimplyDiscus The British and International Discus Keepers Association