Algae bloom during tank cycling near window

You've got a double problem right now — your tank is cycling, which already means elevated ammonia and nitrite acting as algae fuel, and a south-facing window is pouring in direct sunlight for hours a day, which is basically an algae growth accelerator. The good news is this is fixable without starting over. Before we sort out the algae, tell me: how far into the cycle are you, are you doing a fish-in or fishless cycle, and what size is the tank?

Three weeks fishless with liquid ammonia — you're doing it properly, which means the green water is almost certainly a phytoplankton bloom triggered by the light, not a sign anything's gone wrong with the cycle itself. The nitrogen cycle is still ticking along in the background. The priority is cutting the light without nuking your beneficial bacteria. Have you got any way to block or shade the window side of the tank, even temporarily with cardboard or a blind?

Understood. In that case I'd recommend API Algaefix — it's a water conditioner specifically formulated to clear green water algae, and it won't interfere with your ammonia-dosing cycle or kill off the bacterial colonies you're building. For a 60 litre tank one treatment typically clears green water within 24 to 48 hours. You'd dose it, do a 20% water change the next day to remove the dead algae before it spikes your ammonia, then re-dose your ammonia back to target. Longer term you'll want something physical on that window — even a cheap blackout blind or a piece of window film on the tank-facing glass will stop it coming back once your fish are in.

It won't affect ammonia, nitrite or nitrate readings at all — those tests are detecting chemical compounds, not light or organic matter. The one thing to watch is that as the dead algae breaks down it can temporarily push ammonia up slightly, which is why the water change the day after treating matters. Just re-dose your ammonia back to 2ppm after the change and carry on as normal. Your cycle bacteria live on surfaces — filter media, substrate, tank walls — not in the water column, so they'll be fine.

Yes, if the direct sunlight isn't addressed it will keep coming back — fish waste will just give it more to feed on. The simplest fix once the tank is up and running is to put a piece of frosted window film or a UV-blocking film on the window itself, or hang a light-filtering blind. Tanks do fine with ambient natural light, it's the direct sun for several hours a day that causes blooms. Sort the window and you're unlikely to see green water again.