Can a rimless tank carry a foam-and-stone riverbank?

You've seen it in a store or on your phone: a rimless tank with water so clear the glass almost disappears. Now you're picturing a riverbank inside it, with stone sloping up out of the water, wood at the edge and plants rooted at the waterline. Then you wonder whether that bare glass, with no frame around it, can take the load. Yes, a rimless tank can carry a foam-and-stone bank. It works if the glass is thick enough for the tank's size, the tank sits on a flat, clean surface, and the foam is fully cured and sealed before any animal goes in.

Short answer: yes, if the bottom and the foam are right

"Rimless" is a style. It doesn't tell you how strong a tank is. Whether your tank can carry a bank depends on three things:

1. Glass thickness. It has to suit that tank's height and length.
2. What's under the tank. You need a flat, rigid, spotless surface, usually with the maker's leveling mat.
3. The foam. It has to be fully cured, sealed, rinsed and tested before water and animals touch it.

Get those right and the bank itself is the fun part. Foam is much lighter than the rock it imitates, and that's a real reason to build the bulk of a bank from foam instead of stacking stone. I can't give you a reliable number for how much lighter a finished bank is, though, so don't treat foam as weightless. A finished bank still has foam, cement, epoxy, real stone and wood in it, and all of it sits on the bottom pane.

Fall is a good time to start. One of the methods below needs about three weeks of curing before water can touch it. Start now and the tank can be running well before Thanksgiving.

Why rimless is different

A framed tank has a plastic rim around the top and bottom. The bottom rim lifts the glass slightly and spreads the load around the edges. A rimless tank doesn't have that. The bottom glass sits right on the stand or mat, and nothing above it braces the panels.

That puts all the stress on the bottom pane and on whatever is under it. Ultum Nature Systems (UNS), a well-known rimless brand, warns in its FAQ that "even a microscopic grain of sand on a stand can create concentrated stress points." A dent in the stand, a high spot in the wood or a speck of grit means the full weight of water, substrate and hardscape presses on one tiny area of glass. Add a heavy bank on one side and that pressure goes up on that side.

Glass thickness is the maker's answer, and it scales with size, especially height. Here's what UNS uses:

• UNS 60S: 60 x 36 x 18 cm, 10.2 gallons, 5 mm glass
• UNS 60U: 60 x 36 x 36 cm, 76 liters, 6 mm glass
• UNS 120M: 47.24" x 23.62" x 17.72", 85.5 gallons, 12 mm low-iron glass

The 60S and 60U have the same footprint but different glass, because water pressure rises with depth. The longer, taller 120M jumps to 12 mm. You'll also find online rimless glass calculators, and they don't agree with each other. Don't use one to decide whether a tank you already own is "thick enough" for extra weight. Use the maker's spec for that exact tank.

Stand, leveling mat and what to ask the maker

Since the bottom pane rests right on the stand, the stand has to be flat, rigid and clean. Check it with a long level along the length, the width and both diagonals. Wipe it down right before the tank goes on.

Most rimless makers include a foam leveling mat. UNS ships a custom-fit black foam mat with its tanks and says it "absorbs micro-vibrations, levels surface irregularities, and protects against glass stress points." A mat evens out small flaws. It won't fix a stand that's warped, sagging or out of level. That has to be fixed before the mat goes down.

The tank listings above don't give stand requirements, and makers vary. Before you add a heavy bank, email the maker and ask three things:

• What stand do you require or recommend for this tank, loaded?
• Should I use your mat, and does using or skipping it affect the warranty?
• Is this glass rated for heavy hardscape on top of normal water and substrate?

Get the answers in writing and keep them. That covers you if anything goes wrong later.

Building the bank: foam, stone and sealing

There are two good ways to build the bank. You can also mix them.

Option 1: Pond & Stone spray foam. Great Stuff Pond & Stone is a foam adhesive made for water features. A distributor's listing says it "is safe for fish" and "cures in 8 hours." Use it to glue stone and wood into place, fill gaps and shape a slope. Shake the can for at least a minute before using it. The foam is flammable while it's coming out of the can, so work with gloves, eye protection and good ventilation, and keep it away from flames. The 8-hour figure is the minimum. I'd give it extra time before it goes near water.

Option 2: Carved polystyrene with a cement skin. This is the method for a sculpted, stone-look bank or back wall. Gensou, an aquarium shop in Singapore, publishes a step-by-step guide:

1. Carve the shape from 50 mm polystyrene.
2. Brush on two coats of portland cement slurry, about 2 hours apart.
3. Let the cement cure for about two weeks. Mist it every 12 hours for the first week, then let it air-dry.
4. Paint it.
5. Seal it with two coats of two-part epoxy.
6. Let the epoxy cure 7 days before any water contact.

The guide gives two tests before you trust the piece:

• Vinegar test on the cement. If vinegar bubbles on it, the cement needs another week.
• Sample test. Soak a 10 cm corner sample in dechlorinated water with a few shrimp for 48 hours.

Shrimp are sensitive, so a sample that keeps them fine for two days is a good sign. I'd also check the soak water's pH against plain tap water, since the guide warns that uncured cement touching water can throw off tank pH. The guide also warns that spray paints can take about six months to outgas, so stick to paints you know are safe under epoxy.

To mount a finished panel, the guide uses beads of silicone at the four corners. Use aquarium-safe silicone, not kitchen or bathroom grades, which can contain mold inhibitors.

Whichever option you pick, the order doesn't change: cure, seal, rinse, test, and only then add animals.

Keep the weight low and the bank where it belongs

The bank should be mostly foam. Use real stone where it matters. Set the heaviest rocks flat on the bottom, low and toward the bank side, so they rest on the substrate instead of hanging off a foam ledge. Keep the bank off the glass seams, too. Don't wedge foam or rock hard into the corners where it presses on the silicone joints.

When it's all in place, do a test fill with no animals. Fill slowly and watch the first days closely. Check the seams, look under the bottom edge for wet spots on the mat, and test the water to make sure nothing is leaching from the bank.

One more thing from my own river tank. I ran the River Tank EcoSystem X-Large kit in a 55-gallon long for years, with water pumped up on the left and cascading down through three pools. Crabs and some reptiles get out of almost anything, so I swapped the light kit for a full screen lid. A riverbank is basically a ramp to the top of the glass, and rimless tanks are often run open. If anything in your tank climbs, plan a lid that fits before the bank goes in.

Checklist

• Get the maker's glass thickness for your exact tank.
• Ask the maker in writing about stand, mat, warranty and hardscape load.
• Level the stand along the length, width and diagonals, and fix any warp.
• Wipe the stand clean and lay the maker's mat.
• Build the bulk of the bank from foam, not stacked rock.
• Spray foam: shake the can, wear gloves and eye protection, ventilate, and let it cure fully.
• Carved panel: cement cures about 2 weeks, then paint, two epoxy coats, and 7 more days before water.
• Run the vinegar test and the 48-hour shrimp sample test.
• Use aquarium-safe silicone only.
• Put heavy stone low and keep the bank off the seams.
• Test-fill without animals, watch for several days and test the water.
• Fit a lid if anything in the tank climbs.

Sources (5)

• UNS 120M 85-gallon ultra-clear rimless aquarium, Buce Plant
• UNS 60S 10-gallon ultra-clear rimless aquarium, Buce Plant
• UNS Aquarium Standard 60U, Aquasabi
• Dow 283064 Pond & Stone foam adhesive, Spray Foam Systems
• DIY aquarium background guide, Gensou