I have small gravel in my tanks, but I don't think it's a necessity. I looked around the i-net to see if this particular question has already been addressed...hope no one here takes offense for my posting it. I found this item by Mark Massingill in TheUrbanFarmingGuys community discussion...seems to cover a lot of bases...I'll paste most of an answer and also give the link:
"...you do not need any gravel in a breeding tank. A clay flower pot on it's side and a few pieces of 4" pvc or fittings for the females to hide in are helpful, but nothing is necessary. The males dig a burrow into the side of a river bank in nature to create their nest area, the flower pot is a substitute for that and they will use it as such.
Depending on the size of your breeding tank it's best to have several females to one male. I have 6 females and 1 male in my 90 gallon breeding tank. I guess that if you had a larger tank you could have more males and females. I found that when I had more than one male, even though I had two flower pots in the tank, they would fight each other over territory.
I have two nursery tanks linked to it that I transfer female with eggs into until they hatch out.
Temperature is important when you want them to breed. The eggs need warm water to hatch out in, and the warmer it is the faster they hatch. The females will not eat while brooding, so you want the eggs to hatch swiftly. It will take them about a week in 85° water to hatch. Also, the females won't lay unless the water is warm enough, I've heard anything above 80° works, but I have the best luck with my tanks at 85°. Once a females eggs have hatched she will breed again in 5 - 6 weeks.
I posted a few pictures on my page here Mark's Page if you want to look at them to see my set up. I did put a little gravel in my breeding tank, but it was so the pvc and flower pot wouldn't roll around, not because the fish need it to breed. They do play with it a lot, picking it up and moving it around so I think they like it, but you don't have to have it."
Hope that helps.