Brown algae in new tank during cycling

That brown coating covering your glass, gravel, and decorations is almost certainly diatoms — brown algae that's completely normal and actually expected during a new tank cycle. It looks alarming but it's a sign your tank is progressing through the nitrogen cycle, not that something has gone wrong. Before we sort this out properly — how far into your cycle are you, and what size tank are we talking about?

You've done everything right — that's exactly what a cycling tank looks like at two weeks. Diatoms feed on silicates that leach out of new substrate and decor, and they thrive in the low-light, high-nutrient conditions of a cycling tank. The good news is they're completely harmless and almost always disappear on their own once your cycle completes and your beneficial bacteria establish properly. Are you cycling with ammonia, or are you doing a fish-in cycle? And have you been testing your water — do you have ammonia or nitrite readings at the moment?

Perfect — fishless with ammonia is the best way to do it, and at two weeks with ammonia still high and no nitrite spike yet, you're in the thick of it. The diatom bloom will likely persist until your cycle finishes, which could be another 2–4 weeks. To actually speed the whole process up — which is the most useful thing you can do right now — I'd strongly recommend API Quick Start. It's a bottled bacteria product that contains live Nitrospira bacteria, which are the real nitrogen cycle bacteria (unlike some older products). Dose it according to your tank size, keep adding ammonia to maintain 2ppm, and you should see that nitrite spike arrive much sooner. Once your cycle completes and you add fish, the diatoms will typically be grazed away or just fade out within a few weeks as light and nutrient conditions change.

Leave them alone during the cycle — scrubbing stirs up debris and doesn't solve anything since they'll come straight back while conditions remain the same. Once your cycle is done and you have fish in, a simple algae scraper on the glass is fine, but it genuinely isn't worth the effort right now. On API Quick Start: the mixed reviews usually come from people who used it as a shortcut without maintaining ammonia levels — the bacteria in the bottle still need a food source to colonise and survive. If you keep dosing ammonia consistently while using it, it's genuinely effective and will meaningfully shorten your cycle. It's widely available on Amazon UK and it's the product I'd pick over Tetra SafeStart or similar for a fishless cycle.

Exactly that. Keep ammonia at 2ppm, dose the API Quick Start today, and test every two days. When you see ammonia and nitrite both drop to zero within 24 hours of adding a 2ppm ammonia dose, your cycle is complete and you're ready for fish. The diatoms will take care of themselves — most people find they've almost entirely vanished within a couple of weeks of adding fish. You're on track and doing this properly.