I am trying to breed discus in a 30" x 12" x 22". I am getting a pair in the mail today, they will be around 4", along with 12 cardinal tetras and 12 hatchetfish. Is that way overstocked? I am going to put 2 filters on and do a lot of water changes. I also have a 15 gallon planted to raise the fry. Any tips to get them to start spawning?