Sand. The "crap" settles on top and can be siphoned of fast, versus being stuck UNDER the gravel, which is harder to clean. You may suck up lighter particles but as long as you don't push the siphon into the sand you should be able to only suck up the lighter pieces of sand and the debris.
Also, Loaches have barbels which can be worn away by gravel, so sand is better for them.
Sand is best, IMO
, its in all my tanks and always will be.
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