The short answer
The easiest way is a magnet cleaner or a blade scraper run down the inside of the glass — you can do it with the tank full, no draining needed. Green film on the glass is one of the most normal, harmless kinds of algae; a quick wipe as part of your routine keeps the front pane clear. Slowing how fast it returns is about light and nutrients.
Cleaning the glass
- Magnet cleaner: the tidiest option. One half sits inside the tank, the other outside; slide the outer magnet and the inner pad scrapes the film off without you getting your hands wet. Great for regular upkeep.
- Blade scraper: for stubborn spots or hard green algae, a scraper with a stiff blade shears it off. Use a plastic blade on acrylic tanks to avoid scratching; a metal blade is fine on glass.
- Do it before a water change so the loosened algae gets vacuumed out with the old water.
Slowing the regrowth
Green algae feeds on light and nutrients, so the levers are:
- Trim the lighting. Long or overly bright lighting fuels algae. Shortening the photoperiod is one of the most effective changes.
- Keep nutrients down. Regular water changes and not overfeeding keep the nitrate and phosphate that feed algae in check.
Tip: a magnet cleaner lives in the tank and takes ten seconds, so you'll actually keep on top of it. Wipe the glass at the start of each weekly maintenance session and it never gets a chance to build up.
Tools and next steps
A good magnet cleaner makes this a ten-second job, and a gravel vacuum removes the loosened algae during your water change. For the wider algae picture see how to get algae off aquarium glass, browse maintenance gear, and test nutrients with a water test kit.