Reds will start not spawning until they feel comfortable about their surroundings, and you're the one to achieve that for your fish.
Once they start showing breeding behaviour (ie. pairing off, starting to build a nest, agressive territorial behaviour, darkening of coloration), you could take them out, and put them in a tank of their own (I guess 50 gallons will be suffcient, because they can do whatever they want without being interrupted). There's a very real chance they'll continue with their breeding behaviour quickly after they've been moved.
Once you feel like you have enough fry (made enough money
), you could place them back with the others, and just let them be...