getting a predator tank without cycling is asking for ammonia trouble, literally the worst fish you can get for a in-fish nitrogen cycle.
If I misunderstood, and you mean that you cycled,
1. sea-hares need lots of lots of food and will spit poison and kill your tank if in a predatory tank, so dont do that
2. Do some cleaning yourself, you need to in a predator tank, your nitrates are sky high and a water change won't stop it, follow better feeding practice, if CuC are deing very fast they won't help clean, just make things worse, gravel vac your tank at least once a week
3. if its a predator tank its likely pretty large, and there is likely sump space for a fuge or algae reactor to lower nitrates and phosphates
4. get copepods or amphipods as they will reproduce much faster than CuC, be eaten less and eat more in a tank where CuC dies fast