Acrylic has a peculiar nature about it and most people are not aware of it and doubtful of this explanation when first heard. Acrylic absorbs moisture. Therefore the side of your lid that is toward the water absorbs water and expands and the upper dry side does not. This is causing the bowing you are experiencing. The way to avoid bowing from this phenomenon is, like Wally and David suggest, put a vertically glued brace lengthwise down the center.
A little more about Acrylic. First, Acrylic and Plexiglas are exactly the same thing "brand" named by two different companies that make it. Second, there are two grades of Acrylic/Plexiglas. Acrylic/Plexiglas comes in extruded and cell cast versions. Extruded is the cheapest and is less appropriate for aquariums but OK for
sumps. It is softer and the surface has irregularities, sometimes visible to the trained eye. This is more likely what
HD offers. Cell cast is cast in a finely machined mold and pressed under great pressure to give more rigidness, hardness to the surface and a finer finish. Cell cast is more scratch resistant and the preferred material for aquariums. Also costing more.
When making
sumps and
refugiums the cheaper extruded Acrylic is fine because any partition glued front to back will stiffen up the unit considerably.
When buying Acrylic aquariums, be wary of cheap prices from unknown manufacturers. At an
IMAC show in Chicago a few years ago there was a truck load sale of really cheap Acrylic aquariums. Prices were unbeatable. But they sold few because it was obvious to many, not all, that something was wrong. The quality of workmanship wasn't there either and was obvious. I've never seen them back at subsequent
IMACs. Buyer beware, you usually get what you pay for in this world.
When you are in the market for a new Acrylic aquarium, search for the best price and then question the manufacturer as to the type of Acrylic material used. The unfortunate thing is people will lie and most consumers cannot tell the difference, at first glance, between extruded and cell cast material.
Feel free to contact me for assistance on this matter. Or any other matter for that matter. That's a lot of matter, isn't it?
