I have loaches (Cobitis) in my 15 gallon and I'm planning to change to a sand substrate because they really love to dig. I'd like black, or at least dark sand, any recommendations?

Tahitian moon sand is supposed to be really good though Ive not tried any of it.I am wanting to try it myself so hopefully one day I can.Susan uses black blasting sand in her tanks,but I dont know how well a loach will do with it.Should be fine but Ill let her tell you that.

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I only use and recommend Estes' Stoney River (it is also known as Estes Marine Sand and Estes Ultra Reef). It does not alter any chemistry and is very uniform in size, the perfect size. It comes in black and I usually use either solid black or a half and half black and white mix. I have been using it for years and have not seen anything else as uniform, as perfectly sized, or with these color options.

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Stocking has NOTHING to do with tank volume, it is all about water changes.

Blasting sand would work, but I wouldn't buy it for a 15 gal tank as it only comes in 80lb bags.

I would go for one of the aquarium sands where you can get smaller bags.

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lol, good point. I'm not sure if the same blasting sand is available here in Portugal anyway.

I have seen the Tahitian moon sand recommended other places, so maybe I'll see if I can get that.

Do I have to make sure it's not sharp for the loaches? They love to dig in their noses, suck stuff up and spit it out through their gills.

Yes as you don't want it to wear down their barbels or cut the noses.

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SUCCESS IS MEASURED BY YEARS, NOT MONTHS!!!
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Ok, sorry for being thick here but how do I know if it is soft enough for them? I'm thinking I should start b looking at it then rub it between my fingers. But just how soft does it have to be? And is most sand sold for aquariums soft enough or will I need to look around or buy a specific brand?

Mine are on the Estes with no issues at all. I have had cories, loaches, eels, scaleless catfish, even amphibians and more on it and never had an issue with any of them.

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Biologist-Professional Aquarium Care-Published Author (Aquarium Fish International Magazine)

Stocking has NOTHING to do with tank volume, it is all about water changes.

I think I've seen that stuff for sale. Is it fairly course, between a fine gravel and sand? I wasn't sure if a finer sand would be better.

Put a little in the palm of your hand, then rub your hands together, if it fells like its scratching the palm it isn't rounded enough and will have some sharp edges.

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SUCCESS IS MEASURED BY YEARS, NOT MONTHS!!!
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It is a little course for sand, but that is good. It means there is just enough space between the grains for oxygen to get to the whole sand bed. It also means it is large enough to sink quickly when disturbed.

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Biologist-Professional Aquarium Care-Published Author (Aquarium Fish International Magazine)

Stocking has NOTHING to do with tank volume, it is all about water changes.

Perfect, I'll do that, though it could be a problem when bags are sealed, not too sure they will be impressed by me poking holes in all their bags, lol. I get the idea though.

I'm not too worried about the first part because I'm not going to make it very deep plus my 5 loaches plus MTS will keep it well turned over but I was wondering about the getting disturbed part, it seems like fine sand might make the water cloudy. Still I have a feeling the finer it is the more the loaches will love it!

I use fine pool filter sand in a lot of my tanks and it doesn't cloud the water at all.

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SUCCESS IS MEASURED BY YEARS, NOT MONTHS!!!
220 Tank build
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Good to know, do you have fish that dig in it?