How do I swap this substrate safely?

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One reader asks about replacing the gravel with sand in his tank. Bob Mehan advises.

I have a 55 l tank, heated to 23°C, housing Cherry barbs, Endler’s livebearers, Norman’s lampeyes, Otocinclus catfish, Peppered corys, and some Amano shrimp. I set the tank up about six years ago with plastic plants and a gravel substrate, but now I have added live plants and I want to remove the gravel, as otherwise I’m limited in plant choice. I’ve also found out that Corydoras prefer a sand substrate, which is another reason for getting rid of the gravel. 

Could I replace it with a layer of Tropica Aquasoil and then have a fine sand layer on top? This would allow me to grow stem plants and also keep my corys happy. 

My issue here is that this is my only tank, and I’d have nowhere to keep the fish while making this change, apart from an unheated 12 l tank with just plants, and as a student I’m on a limited budget when it comes to setting up anything else. So how would I go about doing this without stressing my fish? Could I temporarily house some of my smaller fish in the 12 l tank or in another container – and how long would this need to be for?

SAMUEL BROWN, VIA EMAIL

Bob replies: Corydoras love soft sand substrates, and will appreciate you removing your current gravel. With sand, they will be better able to exhibit their natural digging behaviours as they search for morsels of food. 

I’d be wary of adding the Aquasoil. Some plant growth substrates can be soft, and when disturbed by digging they can disperse into the water, making it cloudy. A thin top layer of sand will likely soon become mixed in with the Aquasoil, causing similar issues. Many of these planted substrates have a high nutrient content and leach ammonia into the water when first added. This is not an issue once things are settled and your plants are growing vigorously, but it could mean you’d have to leave your tank fish-free for several weeks until things settle down — and even then, disturbance to the substrate by the catfish might cause a spike. 

Corydoras require fine substrates.

I recommend a sand-only substrate with some of the hardier, easier to grow rooting species like Cryptocoryne, available in a wide range of species and colour forms. With weekly feeding most of these plants will do very well without the need for nutrient-rich substrates.

If you do decide to go ahead with your original plan to use Aquasoil, it is relatively easy to make a temporary, budget-friendly aquarium to house your fish in for a few weeks, using a large, plastic storage box. Your problem will be that you will need to use your current, mature filter and heater to keep this temporary housing going but you’ll also need both items to mature the new tank.