This is a specific conversation about Fish aquarium setup — getting started right — ask about yours and get one honest answer for your exact situation.
inparticular.ai
That's a genuinely tricky situation — if your tap water is already reading 40ppm nitrate straight from the source, you've lost the most reliable signal that your cycle is complete. Normally you'd watch nitrates climb then hold while ammonia and nitrite drop to zero, but when your baseline is already at 40ppm, that endpoint becomes invisible. Before we talk about anything else, can you tell me: what size tank are you cycling, and what fish are you planning to keep? That'll shape the solution.
It's a 120 litre tank. I'm about 3 weeks into a fishless cycle with ammonia, but every time I test nitrate it reads 40ppm regardless of whether I've done a water change or not. I want to keep a community of tetras and maybe some corydoras.
inparticular.ai
Good news: tetras and corydoras are a great match and neither is particularly sensitive to nitrates at moderate levels, but 40ppm as a permanent baseline is still too high for long-term health. The core problem here is your source water. For a 120 litre community tank, the most practical fix is blending your tap water with RO water — reverse osmosis water has zero nitrates, so mixing roughly 50/50 brings your starting point down to around 20ppm, which is manageable and finally gives you a readable window to confirm your cycle. Are you currently testing with a liquid test kit or strips?
Liquid kit — the API Master Test Kit. So I need to buy RO water? Can I make my own?
inparticular.ai
The API kit is exactly right, so your readings are trustworthy — that confirms the issue really is your tap. You can absolutely make your own RO water, and for a 120 litre tank it's worth the upfront cost. The product I'd recommend is the Aquatic Life RO Buddie — it's a compact 4-stage reverse osmosis unit designed for aquarium use, widely available on Amazon UK, and it produces water with effectively zero nitrates, chloramine, and heavy metals. A 50/50 blend with your tap water will bring your nitrate baseline to around 20ppm. Once you see that figure hold steady after a water change rather than climbing, you'll know your cycle is running properly. The unit pays for itself quickly versus buying RO water by the container.
That makes sense. But while I'm waiting for the RO unit to arrive, is there anything I can do to manage the nitrates in the meantime?
inparticular.ai
Yes — add Seachem Prime at every water change. It doesn't remove nitrates, but it detoxifies them temporarily, which buys your beneficial bacteria time to establish without stressing any future livestock. It also dechlorinates and detoxifies ammonia and nitrite, so it's doing multiple jobs during your cycle. More practically: don't add fish yet. Your ammonia and nitrite are your real cycle markers right now — once both have been consistently at zero for five to seven days after a full ammonia dose, your cycle is done regardless of what the nitrate reads. The RO unit is the long-term fix; Prime is the short-term safety net.
Perfect, that's really clear. So the RO Buddie plus Seachem Prime, then do a 50/50 blend for water changes going forward?
inparticular.ai
Exactly right. RO Buddie for production, Seachem Prime dosed into every change, and a 50/50 blend to keep your working nitrate baseline at 20ppm or under. Once the cycle is confirmed with that cleaner water, your tetras and corydoras will have a stable, healthy environment from day one. Keep dosing Prime with every water change permanently — it's cheap insurance.
