open top tanks are fine..air fresheners and whatever you spray dont affect your tank...my missus is the fragrance queen
and have never had any probs.but bear in mind that IT IS EASIER TO POLLUTE A PUDDLE THAN A POND
My 2ft tank is open top, and a hate it. I'm topping the water levels up by a pint almost every day and there's no heater, just the heat off the lights. I've got a perspex cover, but I need to make cut outs for the filter pipes. That requires work, and I'm far too lazy for schtuff like that.