Doubt it will cause any trouble at low wattage but I did meet a guy whose 1000W heater caused his acrylic to melt which eventually caused water to leak on wires and start a fire in his garage. Mind you his water level had dropped and the heater was laying on the acrylic dry. I think with water around it, there is no way it can cause any real trouble. I mean water can only get so hot. Well, that's actually up for debate but in general it holds true. So, I wouldn't worry about it in your case. Mine I just didn't want my two 300W heaters laying directly on the acrylic so I put vinyl around them. It doesn't transfer heat well and gives room for the water to flow around the heaters.