If you wait until she is actually dropping eggs, you can pull them out of their respective tanks and put them into the prepared tub. Once she is done - usually an hour or less - just place them back in their tanks. I usually then drain the spawning tub down to just a 1/2 inch of water or less and refill it with nice, clean water. Set up your heater and sponge filter going - to season - and wait for the fry to hatch.
If you have both parents in a tank you do not wish to raise fry in, simply place a spawning mat - a mat of plastic plants if you wish - on the bottom of their tank. When she is done - remove the mat. I rinse it gently in prepared water and place it in it's separate tank - for hatching.
When two fish spawn, they make a mess of the water parameters. Either way you do it, you should clean the water they use to spawn in.
A fry tank, if started out with fresh water will need to be watched carefully. They have a tendency to go to ammonia as the fry hatch out. Careful water changes and all will keep it under control - and then the sponge filter will kick in and the beneficial bacteria will grow as the fry grow.