Getting back to the original question about a discus tank:
1. Is the filtration provided by the two HOB's adequate for the "pristine" water conditions that I have read that discus require? I'm unsure of the models, but they take up almost the full 48" of the 90g. If not, what would be the preferred form of filtration?
Discus prefer clean, bacteria free water, so it's more important that whoever is taking long term care of the tank knows that they'll have to do water changes on a fairly regular basis. Discus breeders change up to 100% of the water in their stock tanks daily, but more commonly for adult discus you can get away with 50% changes twice a week or so depending on the tank. Tank bred discus are hardier than their wild cousins, so if you properly acclimate them to your tank, you won't have to worry as much about pH and TDS values so long as the water is clean and changed often. Suggested pH from 6.5 to 7.4, TDS from 50 ppm to 200 ppm should be fine as long as it's stable and clean.
As far as the filters, aim for a ten tank volume per hour turnover rate based on the flow ratings of the filters. Don't worry, discus can take up to a moderate current in their tanks with no problems. I have over 500 gph on one of my 42 gallon discus tanks and they're hardly phased by the current.
2. How much lighting do I need in a tank of this size/depth for "low light" plants? What type of lighting? Do we need to change the substrate to something finer than pea gravel?
That depends on the plants your trying to grow. If you are going to use finer rooted plants, you might have to change out the substrate to one more suited to plants such as eco-complete, flourite, or any of the laterite based substrates for aquatic plants. You'll need enough for a minimum 3-4" depth. As far as wattage is concerned, 2-3 watts per gallon for low light plants is a rough guestimate. You may have to replace the tank hood with a glass canopy and a different light fixture, such as a compact florescent lamp or T5 lamp.
3. For the small schooling fish, I was considering rummy nose tetras. Are they able to thrive in the warm water that discus prefer? Are there any other schooling fish that would do well? (Probably a school of 10-15 for the tank.) I'm going to try to talk her into giving the gouramis away and keeping the tank to discus/tetras/some sort of algae eaters. Should we keep the pleco?
Sure, rummynose are fine and can take discus temperatures. Scissortail tetras and silver tetras are also nice additions. I've had good luck with zebra danios as well in discus warm waters. You can keep the pleco, it should be able to take the heat, but if not bushynose plecos and rubber lipped plecos do well in discus tanks.
4. Anything else I should know about discus tanks? Things that have/have not worked for you
First and foremost, they're cichlids. This means they will establish a definite pecking order with the first fish in her tank most likely becoming the dominant fish because it's been in there the longest. I'd suggest removing it to a small 20 gallon tank while you re-decorate the main big tank and then introduce all of the discus at the same time, giving all the fish an equal footing for establishing the new pecking order. Deaths are rare if you give the alpha fish plenty of targets to chase around, so discus should really be kept in groups of five or more fish, or just one fish solo.
Discus are pickers, and they will graze the bottom of the tank and the plants for bits of food to eat. This poses a problem if you don't keep the substrate clean, which is difficult to do in a planted aquarium. At high concentrations of bacteria, discus can develop bacterial HITH (hole in the head), the damage is unfortunately perminant and scarring. In fact, most breeders don't have any kind of substrate at all in their discus tanks to keep bacterial loads down and make cleaning easier.
Keep them warm, discus tanks should be maintained between 84-86 degrees f..
Discus are also known to secrete a fear pheremone into the water like other schooling fish. They can dash themselves against the sides of the tank if startled. Water changes and time will allow them to calm down. Sometimes this takes weeks or longer.
