I understand that you know about cycling a tank and allowing the nitrogen cycle to complete, but have you kept Discus before? They're remarkably sensitive to elevated NO3 levels and disease. A planted tank is a great environment for Discus, as NO3 levels are usually low with fast growing plants present. But that's why I recommended you wait several months for the plants to take root and start growing steadily.
But I'll assume for now that you are familiar with Discus husbandry and focus on your plant question...
Before we recommend plants, we need to know more about their proposed environment. Discus aren't fond of bright light, though, so I am going to assume you're going to be setting up a moderate-light planted tank. If your tank is a standard 90 AGA, it's probably 48" x 18" x 25". I don't know if you've already planned your lighting strategy, but a set of two 55w PC lights from www.ahsupply.com
, supplemented with two 40w NO flourescents with GE Plant & Aquarium bulbs would be ideal. Just over 2 watts per gallon, which is enough to grow an excellent variety of plants, without creating a stressful environment for the Discus. Even with that much light, a good, tall background screen of stem plants would be ideal to creat some shade for the fish.
The stem plants I've had the most success with are:
- Giant Hygro: Tall stems with long, broad bright green leaves. Grows very quickly in bright light, even does well under moderate light.
- Sagitaria Subtulata: Tall, long dark green leaves. Should do well with the light levels and will grow along water surface to create shaded areas.
- Rotala Indica: May not be as prolific with 2wpg, but should still do fairly well, as long as you supplement CO2.
Which brings me to the next question - what type of CO2 supplementation have you considered? You can grow plants without supplementing CO2, but will most likely be frustrated with the slow growth.