High phosphate tap water ruining your tank cycle?

Algae going absolutely wild during your cycle is a strong signal that your tap water is high in phosphates — it's a really common issue in certain UK water areas, and it makes the cycling phase genuinely miserable because algae just feasts on all that phosphate while your tank is nutrient-rich and unsettled. Before we talk solutions, let me make sure I understand your setup: what size tank are you cycling, and have you actually tested your tap water for phosphates yet, or are you going on the algae as your main clue?

Good news: fishless cycling with ammonia is exactly the right approach, so your method is sound. The green tank almost certainly means high phosphates in your tap water — algae spores are everywhere, and phosphate is the fuel they need to explode like that. A 120 litre tank is a good size where this is very manageable. First thing I'd do is actually test your tap water with a dedicated phosphate test kit, because knowing the number tells you how aggressive a fix you need. Do you know roughly what fish you're planning to keep once the cycle is done? That matters for how thoroughly we need to solve the phosphate problem long-term.

Tetras, corys and livebearers are all solid choices and none of them are particularly phosphate-sensitive themselves, but you're right that the algae situation needs sorting before you add fish — not because it's immediately dangerous, but because it signals your water quality starting point is poor, and it'll keep being a battle if you don't address the source. Here's what I'd recommend: get the API Phosphate Test Kit to confirm your tap levels, and then use Seachem PhosGuard in your filter. PhosGuard is a silica and aluminium oxide media that physically pulls phosphate out of the water column — you put it in a mesh bag in your filter and it works passively. For a 120 litre tank you'd use roughly 80–100ml of it. It's widely available on Amazon UK, it's reef-safe so it's gentle chemistry, and it'll bring your phosphate down within a few days. Once phosphate drops, the algae stops getting new fuel and your existing algae starts dying back. Have you got any mechanical filtration in your filter that we can put the PhosGuard behind?

That spare tray is perfect — put the PhosGuard in a fine mesh media bag in there, after the mechanical sponge so it stays cleaner for longer. On water changes: yes, keep doing them as part of your cycle, but here's the thing — if your tap water is the phosphate source, every water change is adding more phosphate back in. So either use the PhosGuard in a small reactor or bag and run some tap water through it before it goes in the tank, or do smaller, more frequent water changes rather than large ones to minimise how much phosphate you're importing each time. Once you know your tap phosphate number from the test kit, you can judge how careful you need to be. Some areas have tap phosphate of 0.5 ppm, some are over 2 ppm — the fix is the same, just the scale differs. The PhosGuard in your filter will handle what's already in the tank, and being thoughtful about water changes handles what's coming in. Your cycle should still complete normally — the bacteria you're building don't care about phosphate.

A bit of both, honestly. Once phosphate drops, the existing algae loses its food source and starts dying back, but it won't vanish overnight — you'll want to do a good manual scrub of the glass and any hard surfaces before you add fish, partly for aesthetics and partly because a lot of dying algae decaying at once can spike ammonia slightly. Your corydoras will actually help with algae on substrate and décor once they're in, and if you add a small group of otocinclus catfish — they're perfect tankmates for the community you're planning — they're brilliant at keeping algae in check long-term. But get the phosphate sorted first, scrub what you can before fish go in, and you'll be in good shape. The main thing is you've identified the problem early rather than after you'd added fish, which makes this much easier to fix.