Short answer: Most sump baffles and refugium dividers are 1/4 in. cast acrylic, cut about 1/16 in. narrower than the sump's inside width so they slide in and can be glued or siliconed in place. Use 3/8 in. for sumps wider than about 24 in. or baffles taller than 14 in. Cast acrylic is the right choice for anything that lives in saltwater or freshwater.
What these parts do
A sump is a second tank under the display that holds equipment and extra water volume. Baffles split it into chambers: water flows over one baffle, under the next, and back over a third. That pattern traps bubbles, holds the water level steady around a protein skimmer, and keeps the return pump chamber from running dry. A refugium divider walls off a section for macroalgae and pods, and a frag rack holds coral cuttings off the glass.
All of these are flat panels with a few notches, slots, or holes, which is exactly what we cut well. They're non-structural: they don't hold back the full head of water, only a small level difference between chambers.
We don't build structural tanks. Aquarium walls and bottoms carry real water pressure, and a failed seam empties the tank onto your floor. We cut the internal parts that sit inside a tank or sump; for the tank itself, use a dedicated aquarium builder. If you're planning one, how thick aquarium acrylic needs to be explains the numbers.
What size and thickness to choose
Measure the inside width of the sump at the height where the baffle will sit. Glass sumps often taper slightly where the silicone fillets are, so measure at the bottom and the top and use the smaller number. Then subtract about 1/16 in. (1.5 mm) so the panel drops in without forcing.
| Sump inside width | Baffle height | Thickness |
|---|---|---|
| Up to 18 in. | Up to 12 in. | 1/4 in. |
| 18 to 30 in. | Up to 14 in. | 1/4 in. or 3/8 in. |
| Over 30 in. | Any | 3/8 in. |
| Frag rack grids | — | 1/4 in. |
For an over-under-over set, the "under" baffle usually stops 1 in. above the floor and the "over" baffles stand 1 to 2 in. below the water level you want to hold. The gap between baffles is typically 1 in., wide enough that bubbles rise out before reaching the pump.
Why cast acrylic, and is it safe for fish?
We cut aquarium parts from cast acrylic. It machines and bonds more cleanly than extruded, and it's the same family of material used to build acrylic aquariums. Once the protective film is off and any cement has fully cured, plain clear acrylic is inert in fresh and salt water. Let solvent-cemented joints cure at least a few days and rinse everything before it goes in. More detail in is acrylic safe for fish and reptiles.
Skip colored or mirror sheet inside the water. It's not that it's known to be harmful, but clear cast is the simple, proven choice, and you want to see through baffles to spot detritus and snails anyway.
Notches, holes, and details we can add
- Teeth or slots along the top of a baffle to break up flow and catch macroalgae before it reaches the pump.
- Cord and tubing notches in the top corners so heater cords and dosing lines pass over without lifting the lid.
- Drilled holes in a divider for flow, or for bulkheads in a separate panel (tell us the bulkhead's hole size, not the pipe size).
- Frag rack grids with 1 in. to 1.5 in. plug holes, plus small feet or notches to hook on the tank rim.
- Sock holder plates with round cutouts for 4 in. or 7 in. filter socks.
Send a sketch with dimensions, or a DXF/SVG, through upload a drawing. Simple rectangles you can price yourself in the instant quote.
Installing baffles
In a glass sump, hold baffles in with aquarium-safe silicone: a bead on both sides of each edge, then 24 to 48 hours to cure before water goes in. Silicone doesn't bond to acrylic as well as it does to glass, but in a baffle it only needs to seal and locate, and that's enough. In an acrylic sump, solvent-weld baffles in place with a water-thin acrylic cement; see how to glue acrylic. Many people prefer removable baffles that slide into U-channel, so they can change the layout later.
Questions
How much smaller than the sump should a baffle be?
About 1/16 in. narrower than the inside width at its tightest point. That leaves room for a silicone bead and for sheet thickness variation, without a gap big enough to bypass flow.
Can I use extruded acrylic for sump baffles?
It will work in many cases, but cast acrylic cuts and bonds more predictably and is less prone to crazing near cemented joints. For a part that sits in water for years, cast is worth it.
Does silicone stick to acrylic?
Not as strongly as to glass. It's fine for sealing and holding baffles in a glass sump, but acrylic-to-acrylic joints should be solvent-welded.