Construction and Materials

Glass is hard and resists scratches. It's heavy, and it's brittle: a hard knock can crack or shatter it. It stays clear over time.

Acrylic is a clear plastic. It's much lighter than glass and resists knocks better, and thick acrylic can look clearer than thick glass. But it scratches easily, it can react with some cleaning products, and it can yellow over time under UV light.

So clean each with tools made for it. Acrylic needs non-abrasive tools and only cleaners its maker allows. A scraper that's fine on glass can scratch acrylic for good.

Standard float glass contains iron, which gives it a green-blue tint. The tint shows most in large and thick panels, and along the edges, because the light passes through more glass.

Low-iron glass is made with very little iron (an iron oxide content of about 0.01%), so the green-blue tint is gone. It's used where clarity matters, such as display aquariums. It's sold under names such as Starphire, Optiwhite, UltraClear and Ultrawhite.

Tempered glass is heat or chemically treated to be stronger. When it breaks, it shatters into thousands of small pieces. It can't be drilled or cut once it's tempered: any holes must be made before tempering. So never drill a panel unless you know it isn't tempered; an overflow box can connect a sump without drilling (Module 12).

Acrylic sheet comes in two kinds. Cast acrylic resists chemicals better and costs more. Extruded acrylic is cheaper and more even in thickness.

A seam is where two panels meet. In glass tanks, the panels are joined and sealed with silicone. In acrylic tanks, the panels are bonded. The seams hold the water in and help hold the panels together.

Check seams as part of routine work. Signs of trouble: silicone peeling away from the glass, gaps, bubbles or channels in the seal, and damp edges or salt crust along a seam.

Never pick, cut or scrape at a seam, even to clean it. Report any change in a seam the same day.

A rimmed tank has a frame around its top edge. A rimless tank has no frame; its panels and seams carry the load. A braced tank has strips of glass or acrylic across the top. A custom-built tank is made to order for one site.

Each design is engineered by its maker. Rims and braces are part of the structure, not decoration. Never remove, cut or drill a rim or brace.

For a custom-built tank, keep the builder's details, drawings and instructions with the system, so anyone can check how it must be supported and used (Module 10).

Rigid PVC pipe and sheet contain few plasticizers and don't become brittle in salt water, even after years. Flexible PVC is soft because of plasticizers, which aren't bonded to the plastic. They can leach into the water, and the tubing turns hard and brittle as they leave. Silicone tubing doesn't change in salt water over time.

Food-safe tells you a material is approved for contact with food. It doesn't tell you how it behaves after months under salt water, light and flow. Choose parts made for aquarium use, or with a long record in reef systems.

A plastic that hardens, cracks or gives off a smell is breaking down. Report it and replace it. The smaller the system, the more any released chemicals build up.

Salt water corrodes most metals. As metal corrodes, it releases metal into the water. Some metals harm animals; copper, for example, harms invertebrates (section 10).

Only metal made for use in salt water belongs in contact with it, as the maker specifies. Titanium, for example, resists corrosion in seawater and is used for heaters. Other metals, such as screws, clips and tools, stay out of the water.

IMARCS rule (Michael Maddox, President of IMARCS): everything in a system is plastic, titanium or 316 stainless steel. Nothing else. Even 316 stainless can pit in salt water, especially in crevices and still water, so check metal parts during routine work (Module 12).

Report rust or corrosion on anything in or over the water, and remove anything metal that falls in.

Silicone needs to cure fully before a tank or sump is filled. Follow the maker's time: aquarium silicone sets within 48 hours. Filling early risks a failed seal.

Epoxy putty, used to fix coral and rock, cures on the maker's schedule; reef epoxy cures fully in 24 hours, even underwater, and its maker warns that skimmers can foam over while it cures.

Cement and homemade rock need curing and soaking before use. Test the pH of the soak water before anything goes near a system. Whatever the material, it isn't ready until it's cured, however dry it looks.

Ozone attacks many materials. Natural rubber breaks down fast; materials such as silicone, Viton, PTFE, PVDF (Kynar) and 316 stainless steel resist it. Anything in contact with ozone must be rated for it (Module 21).

UV light breaks down some plastics, and acrylic can yellow under it. Heat from heaters and lights has limits too: check each part's temperature rating. Chemicals: cleaners and solvents can damage acrylic and some plastics, so use only what the maker allows.

For any part, check the maker's rating for each exposure before it goes into use.

Find out where it came from and what it held: fish, invertebrates, or medicines. A tank or part with an unknown history is treated as unsafe until it has been checked.

Inspect it: seams, scratches, crazing, cracks, corrosion, and any smell. Clean it, then test it with plain water, ideally outside the room it will go in, before it's trusted with animals.

Anything from another system can also carry pests and disease (Module 23).

Copper is used to treat fish, and it's toxic to invertebrates. Copper binds to silicone seals, soaks into plastic equipment and gets into porous materials like rock and sand. It can come back out later.

Removing copper from a system that has held it's very difficult. Treat such tanks as permanently unsuitable for invertebrates, however long ago the treatment was or however they were cleaned.

So keep copper-treated tanks, rock, sand and equipment for fish only, and label them. Ask about medicines before reusing anything (Module 30).

Crazing is a network of fine cracks in acrylic, on the surface or below it. Uneven stress, temperature differences across a panel, and solvents or unapproved cleaners can cause it. Note scratches, and report deep ones so they can be judged.

Other warning signs: seams that peel or gap, panels that bow, creaking, and damp edges or salt crust where there shouldn't be any.

Report any of these the same day. Don't add water, move the tank or try a repair; a person qualified to judge it decides what happens next.