too small for adults
For discus, it all depends on the size of the fish, the volume of water changes and the filtration.
IMO you cannot keep discus for long in 30 gallons. Adults need 10 to 15 gallons per fish, babies grow fast. Discus are better in groups of 6 or so, for they pick on each other as cichlids can do. With 6 fish, 30 gallons is not enough. If you got a mated pair, 30 gallons if tall is great, but a mated pair is probably $300. If you got small fish, you'd need to move up to a bigger tank by about 4 months age, at 2 to 2" size.
I started with 23 fish at 1" in a 20 long, with daily 50% water changes and 2x/day waste siphoing of a barebottom tank. They were moved to a 55 gallon in a few months with daily 50% water changes and dual filters, a Penguin 330 and sponge filters. Then they moved into a 105 gallon with every other day 60% water changes and triple filtration, a HOT 250, the Penguin 330 and a big sponge filter. Then they were split up with 16 remaining there, 5 of the biggest plus the runt moving to a 58 gallon with dual canisters, an Ecco and an Eheim 2026 Pro2. They still get 3x/week 50% water changes. At 13 months they have gone from 1" to 6" (biggest fish), many are 4.5" to 5". All tanks are barebottom and siphoned daily of solid wastes. Fish are fed 3x/day. Temp 82 to 83 degrees. All tap water, pH 8.0, moderately hard.