There are numerous ways to remove algae from the glass in your aquarium. You can remove it physically, chemically, or naturally. All of the following assume you’re removing algae from a glass tank containing water rather than empty. A dry tank or an acrylic tank are a whole different conversation.
Let’s start with the first one – physical removal. Depending on the type of algae and how long it’s been on the glass, the tools needed to remove it will vary. For light algae, non-scratching scrub pads will usually do the trick. There are many varieties of these available. Just make sure they don’t contain any built-in surfactants or cleaning agents, such as soaps and detergents. Also, be careful that the type you select doesn’t degrade easily with friction. In other words, don’t use scrub pads that disintegrate easily, leaving bits of the pad in the tank. Typically, a light scrub from most pads will remove the majority of algae.
For denser algae or algae that is well-established and been on the glass for some time, use a metal or plastic scraper such as putty knives. You’ll want a type that flexes little. Too much flex in the scraper and you’ll either bend it permanently or you’ll work yourself to death scraping. As for size and stiffness, I like the 4″ Tarvol Wall Scraper putty knife (see left photo above). It’s flexible yet stiff and has a nice ergonomic handle. It will absolutely plow through caked green algae without scratching the glass.
If you have to reach into tight spaces and/or are reluctant to use metal on your glass, get a nice stiff plastic scraper. I use the 1.5″ Foshio plastic putty scraper with replaceable blades (see right photo above). The four nice things about this tool are 1) it’s small, 2) it’s got a decent sized handle that is easy to hold, 3) it’s quite rigid, and 4) it comes with 100 replaceable blades.
NOTE: In reality, the 4″ metal scraper is quite a bit larger overall than the 1.5″ plastic scraper. They appear similar in size in the photos but they aren’t. I should have photographed them side-by-side.
To chemically remove algae, you have several options also, depending on the algae type. The chemicals you choose will also depend on whether your tank contains livestock (fish, plants, invertebrates, etc.) or not. There are several commercially available algaecides. You can also overdose with a liquid carbon plant supplement such as Seachem’s Flourish Excel.
For chemically killing algae in a tank with no livestock, you have more options. You can use algaecides without fear of harming any tank inhabitants. Or you can go to the extreme and “nuke” the tank with bleach. The nuke approach is very effective, but I would remove most of the tank decorations and rocks. Many plastic plants will survive a bleaching also without dulling the colors. Also, you can run the filters with bleach. I’ve nuked a QT tank before running a canister filter and just let it go for a few days. I cleared all the bleach just using water conditioner and converted it to a regular tank with fish without any issue at all.
For a natural approach, just keep all the lights on the tank turned off. If the tank sits in a room with ambient light, you can speed up the process by either keeping the lights off or covering the tank completely with a dark blanket, sheets, or similar. Most all aquarium algae need light and will simply die without it. Also, most fish will do just fine without light, but your plants won’t.
The main point is that you have several algae removal options at your disposal, depending on your tank set-up. Just remember, if you have livestock, be sure to use whatever method is best for them. Also, it’s usually a good idea to figure out why you have algae in the first place.
Algae removal and algae eradication aren’t the same thing. The only two 100% effective algae killers in the above are bleach and darkness, and either can create unwanted problems if not implemented correctly.