Dave,
Thanks for the extra info. You think I'd be ok just taking the cat out of the tank? I don't need all of them to make it but would like at least half since they lay quite a few eggs. I also have a pleco. Would he try to get at them also. These fish just seem to stress a lot when removed from their tank so I was trying to avoid that. I could get a devider but have not seen one that looked large enough (I really can't say I've looked really hard since this is my first time dealing with this). I've thought of making one with a roll of screen (plastic screen not metal) and pvc pipe. Or is there another diy method that might be better. Is the ani-fungal something that will work it's way out of the water after a while?
Thanks again and sorry for all the newb questions.
Willy,
No problem. Removing the catfish would help, but still there are other Dempsey's in the tank, and when the fry are free swimming, as staunch defenders as their parents are, their tankmates will attempt to make quick skirmishes towards the cloud of fry around the parents for a quick snack, so you might want to net out as many fry as you can as soon as they are free swimming and raise them in another tank, feeding them live baby brine shrimp for a week or until they show disinterest in the shrimp. Then you can feed regular prepared and frozen foods geared to their size.
You'll need to keep their tank scrupulously clean. Fry tanks are usually bare bottom so its easy to remove debris. Do 50 percent water changes a day and feed them, no more than they can eat in a minute or so, every two hours. Keep their bellies full. That's the only way to get big, healthy fry - clean conditions, frequent feeding, and daily water changes. That, plus space and warmth (use a heater, set it at 77 degrees) will push their growth so they'll be a salable size within 6 weeks.
The pleco won't go after the sleeping fry on purpose, but in their night time roamings it won't say no to tiny fry as a snack. It's very rare for the herbivorous plecos to get rich food like baby fish, so if they run across them, they will eat them.
It'd be tough to have a PVC frame tight enough to the side of the tank that a Dempsey couldn't push through, but you can try it. Use aquarium epoxy to secure the plastic screen to the frame. Make sure the frame is set into the bottom, side to side and above the top of the water.
Buy a 20 or better a 40 breeder and set it up like I described. I use danios to keep the fry tanks cycled and remove them when I have a spawn and start using the Methylene Blue. Larger tank is better because more space gives you better and larger fry. Dempsey fry aren't difficult to raise; there just are a bunch of them from a spawn.
Dave