Switching to sand substrate without crashing your cycle

Switching from gravel to sand is one of the more nerve-wracking substrate swaps because you're right to be cautious — most of your beneficial bacteria actually lives in your filter media, not the substrate itself, so the risk to your cycle is lower than people fear, but there's still a right and wrong way to do it. Before I give you a plan, tell me: what size is your tank, how long has it been running, and what fish are you keeping in it?

Perfect setup for this switch — corydoras absolutely thrive on sand, they'll sift through it constantly and it's much better for their barbels than coarse gravel. And at 18 months your filter is very well established, so your cycle is sitting almost entirely in there, not in that plastic gravel. That means you can swap the substrate without a major ammonia spike if you're methodical about it. One thing to avoid: don't do any filter cleaning in the two weeks around the swap. You want your filter bacteria colony at full strength as a buffer. What's your current filtration — canister, hang-on-back, internal?

An 18-month-old Eheim canister is about as safe as it gets for a substrate swap — that media is absolutely packed with bacteria. For sand I'd go with Unipac Sahara Sand. It's a fine, natural-coloured aquarium sand that's inert so it won't affect your water chemistry, it's soft enough that your cories will love sifting through it, and it doesn't compact as badly as some cheaper sands which can cause anaerobic dead spots. For a 200 litre tank aim for around 15–20kg to get a 3–4cm depth — that's enough for the cories to dig in without being so deep it causes problems. For the actual swap: move your fish to a bucket with tank water and an airstone, remove the old gravel, rinse the sand thoroughly until the water runs clear (this takes longer than you think — budget 30 minutes), add it in, refill slowly using a plate to diffuse the flow, then float your fish back in. Test ammonia and nitrite daily for a week after. You almost certainly won't see a spike, but it's worth knowing.

Honestly, with your setup you don't need it — but if it'll give you peace of mind, a bottle of Seachem Stability dosed for three or four days after the swap does no harm and the bacteria strains in it are actually viable, unlike some cheaper products. What I'd say matters more is having a bottle of Seachem Prime on hand during that first week. If you do see any ammonia tick up, Prime detoxifies it for 24–48 hours while your filter bacteria catch up, which buys you time without an emergency water change. Those two products together are a solid safety net for any major tank change, not just this one.

You're well set — your tank is mature enough that this should go smoothly. The cories will be noticeably more active once they've got sand to work with, it's one of those changes where you see the difference in fish behaviour almost immediately. Good luck with the swap.