There is no harm in having a UV
filter in a freshwater tank. The upside is that it will help kill some algae and some free-floating parasites. The truth is that in a 38 gallon tank you should have no problem doing the maintenance required to keep algae at bay. Simple water changes and proper feeding will do that for you. Quarantining and observing new fish will prevent the introduction of parasites.
If you were planning on putting a UV
on a large pond, very fragile reef tank or a large fish holding system I would say go for it. But for your average FW
tank, its unnecessary.