What a lid is for
A lid does four jobs, and no single material does all four well. It keeps fish in the tank. It controls how much water evaporates. It lets oxygen in and carbon dioxide out at the surface. And it stays out of the way of the light, the equipment and your hands at feeding time. Glass, acrylic and mesh each trade one of these for another, so the right choice depends on which job matters most on your tank.
Glass lids
A sheet of glass, usually on a plastic hinge, is the traditional cover for a framed tank. It seals the top almost completely, which is its strength and its problem.
- Evaporation drops to nearly nothing, so a top-off system has less to do and the room stays drier.
- Gas exchange drops too. The surface is where a tank breathes; a sealed top lets carbon dioxide build up in the air pocket under it and slows oxygen in. On a reef that shows up as a lower pH, on a heavily stocked tank as fish hanging at the surface after lights out.
- Heat is trapped. A sealed glass top under an LED or a hot summer room pushes water temperature up, which is the opposite of what a reef tank wants.
- Light is cut by condensation and by salt creep or lime spotting on the underside, and the loss grows between cleanings.
- Fit on a rimless tank is awkward: there is no ledge for the glass to rest on, so it sits on the bare top edge with clips, and it slides when nudged.
A glass top suits a framed freshwater tank in a cold room where evaporation and heating cost matter more than gas exchange. It is a poor match for a reef or a rimless display.
Acrylic sheets
Clear acrylic is often used as a cheaper, lighter stand-in for glass, and sometimes as the frame of a home-made screen top. It behaves like glass for evaporation, gas exchange and heat, with a few differences of its own.
- Scratches easily. Salt creep wiped off with a dry cloth leaves a haze within months.
- Bows and warps. A thin acrylic sheet over warm, humid air sags in the middle and curls at the edges, and once it has moved it rarely goes flat again.
- Cracks when cut or drilled. Acrylic is brittle, so a feeding hole or a notch for a cord tends to start a crack from the corner. That is the reason we do not cut lids from it.
Mesh tops
A mesh top is a frame with a window of netting. It is what most reef keepers end up with, because it keeps fish in without changing the way the tank breathes.
- Gas exchange is essentially unchanged from an open top. pH and oxygen stay where they were.
- Heat escapes. A fan across the surface still works.
- Light passes through, with a small loss that depends on the mesh. A fine, dark window screen shades noticeably; a clear or white mesh with a wider weave shades very little.
- Evaporation continues almost as if the tank were open, so an auto top-off is still worth having. A removable clear cover over the mesh gives you a sealed top on the days you want one, for example over a holiday.
- Jump protection is only as good as the gaps. A frame that sits on top of the rim leaves a slot at every cord and at the corners; a lid that sits inside the glass does not.
Side by side
| Glass lid | Acrylic sheet | Mesh top | |
|---|---|---|---|
| Keeps fish in | Yes, when it covers the whole top | Yes, when it covers the whole top | Yes, when the frame closes the gaps |
| Gas exchange | Poor | Poor | Same as an open tank |
| Evaporation | Nearly stopped | Nearly stopped | Continues; add a cover when wanted |
| Heat buildup | Traps heat | Traps heat | None |
| Light to the tank | Loses light to condensation and spots | Loses light, and hazes with scratches | Small loss, depends on the mesh |
| Rimless tank fit | Clips on the bare edge | Clips or rests on the edge | Depends on the frame |
| Feeding hole, cutouts | Hard to add | Cracks when cut | Easy in a rigid frame |
| Ages | Stays clear, breaks if dropped | Scratches, bows | Mesh lasts years; frame depends on material |
What we cut, and why
Every MasterLids lid is a frame of 3/8-inch clear polycarbonate with a mesh window, cut on a CNC to the outside of your tank's glass with a lip that sits inside it. Polycarbonate was chosen over acrylic for one reason: it can be cut, notched and drilled without cracking, so a feeding hole, a cutout for a light bracket or a notch for a cord is just another line on the drawing. It is also far more impact-resistant than glass or acrylic, and at 3/8-inch it does not sag over the width of a tank.
The clear frame keeps the look of an open rimless tank. The mesh window gives the gas exchange of a screen top. A removable evaporation cover snaps over the mesh when you want the top sealed, and comes off again when you do not. The lip inside the glass is what closes the gap a screen frame leaves at the corners.
If your tank is a production model, the lid is already drawn: find it by brand. For anything else, measure the outside of the glass and build it from those numbers.