Get a larger tank, that will help.
adding dithers is just going to mask the problem and not solve it, having those fish in a thirty gallon tank is a recipe for disaster so to speak.
if you are trying to grow out your fish relatively fast keep this in mind
no matter how hard you try, the dovii will grow one hell of a lot faster than any festae, and will be a lot more agressive to.
festae's are notorious for their painfully slow growth rate, and dovii are known for their extremely fast one!
here are my tips to get a good growth rate
1. feed a range of staple pellets, occasionally supplemented with fresh or frozen foods (NO FEEDER FISH!)
2. whenever you can do a water chance/ gravel vac DO IT. changing your water often will improve the water quality hence better for the fish
3. expose the fish to as less stress possible (this will be near impossible for you, as the dovii is the source of stress to both of the festae)
ultimately this is my recommendation
choose ONE fish from your current three and keep that one, this way you do not wake up one day to find out that one of your beloved festae has become dovii chow!
