My Zebra Danios (Long finned, Glofish, Red, Golden) took 30C without showing any distress (tank temp in August 2008).
Leaving the temp up for a couple of weeks should suffice if your ick infestation is mild.
However, if you suspect a moderate to severe infestation, huge daily water changes and thorough substrate vacuum could be needed. Hope it's not your case since doing this would likely jeopardize your cycle.
Please look for pics of fish with ick to confirm if yours indeed have this or not.
If you reach 30C, consider increasing aeration with added water surface agitation (e.g. take 2cm of water height off your tank if you have a
HOB filter so its output splashes it harder or simply add airstones or a bubble wand) since dissolved oxygen levels drop dangerously as temp reaches 30C.
Pepe
Santo Domingo