If you added any fish or plants to your tank in the last few weeks, it's possible you added ich. If however you have been clean for 4 months and nothing was added, then you can't have ich. It's life cycle in a tropical tank is far to rapid to have been there for more than a few weeks. (in cold water, it can indeed take a long time to manifest.)
If you have ich, depending on how well your fish handle heat and salt, a common method is heat and salt. I've used this method without fail. Heat and salt will disrupt and kill the parasite over time (although not in the stage where it is visible on the fish.)
See link below.
Basic summary:
1) raise heat slowly to 86, although I prefer 88 as a target to eliminate temperature fluctuations and errors in the thermostat.
2) add salt slowly to raise salinity (see calculations in the link below). Not all fish can handle this, so be sure.
3) add aeration to ensure fish have enough oxygen
4) give the tank 10 days at 88 degrees, but 14 is a more sure time frame
5) be sure to dry out anything that comes out of the tank before putting it back into that or any other tank
6) reduce feeding slightly to reduce stress from ammonia
7) if you see unusual stress or respiration at any point stop adding salt, do add more aeration
8) do not stop the treatment just because you don't see the ich. It will survive past the point you actually can see it.
9)
Do NOT use heat and salt with medicines.
A good siphon of the bottom is a good idea before you start as the ich will often be there in "eggs" waiting to divide and hatch
http://www.monsterfishkeepers.com/forums/showthread.php?20681-One-proven-way-to-treat-ich-ick-%282-Viewing%29
