I have an acrylic cover on my 180 gallon, mostly to reduce evaporation into the house and prevent jumping fish. Some observations:
- The acrylic warps a bit, both due to heat from the MH lighting and absorption of moisture on one side. It still works.
- Acrylic absorbs quite a bit of UV, it's not entirely clear how this is different from glass or whether this is a problem. I got some special UV transparent acrylic and this seemed to make a small difference in coral coloration under my 400W SE MH, but not much.
- Covering the top prevents excessive evaporation, but it also means that the tank is much easier to overheat. I have to run a chiller, and probably would not need to if the tank were not covered.
- Covering the tank discourages maintenance - it's another layer to get off, so any quick little thing now takes more time and effort.
- As for salt creep and cleaning, in my situation this has not been a big issue. I get some condensation when the lights are off, but that is evaporated when the lights come on. Moreover, the condensation is basically pure - not salt water. Not a lot of salt gets on the acrylic, and I have not had to clean it off more than once a month or so. On the other hand, I don't have a whole lot of surface agitation.
--Albert