It takes about 5 weeks running fallow (no fish) to be sure a tank is free from ich or velvet. If you have not allowed this time to pass and the life-cycle of the parasites to complete to their natural extinction, then there could still be parasites on/in the water/tank/rock/substrate...If any rock in the new tank came from the old one the parasite could be in the new one too. CUC cannot keep ich alive, but adding cuc or even some water from a contaminated tank can bring it into yours. running fishless for 5 weeks is the ONLY way to be sure a tank is free &clear of ich. Ich & velvet parasites are just like mosquitos - they need blood to feed on to get the energy to re-produce. Take away all the red blooded creatures and they die out...
I'd wait a month leaving the new tank fishless, before adding fish......Besides - buy a fish now, and put him in qtine in his own tank and treat him (if needed) for parasites...It takes a month+ to be sure a fish is clear, and the same month or more to treat, so while your monotoring/treating a new fish(es) your DT tank is clearing up.
