I think gravel is better in heavily planted tanks 1 to 3mm diameter.
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Gravel that is made for planted tanks is better than sand for planted tanks
Gravel is easier to maintain long term, using a gravel vac/syphon it is much easier to clean than sand, it stays in the tank too! Oh and it doesn't mess up the filter either.
Virtually all of my tanks have been mixed grade set ups over the years, no real problems either way. I will say though, sand disappears over time and needs replenishing, gravel on the other hand can be re-used until you decide to change it, with considerably less accidental/incidental loss over the same time frame.
Some fish have developed habits such as feeding or breeding where sand or silt is used naturally. Replicating this in captivity is good practice on the part of the fish keeper. The same goes for rocks/stone/gravel and wood.
Most ailments that develop in fish are a result of dirty substrate, maintenance by the fish keeper rarely comes into question by said fish keeper who normally finds an alternative cause than themselves, "must be the gravel" etc. (includes barbel erosion, underside skin lesion, fungal infection etc. etc.)
I heartily disagree with those statements. I have sand in my heavily heavily heavily planted high tech tank and it is working amazingly well. I tried flourite in my 10g high tech planted and absolutely hated it with a passion.
Sand imho is much better for planted tanks solely for the reason that plants take root much easier in sand than they will in any type of gravel. When I used both the flourite recently and small grain gravel I had when I started keeping fish it was a constant battle trying to get plants to stay down. That all disappeared when I switched to sand.
The primary reason that people choose sand over gravel is that sand is much much easier to clean. It will never trap debris like gravel will which can and generally will cause issues with nitrates in a tank.
All you need to do to clean sand is to hover the gravel siphon an inch or so above the substrate and allow the debris to get sucked up naturally. As for damaging / clogging filters, the intake should never be anywhere near close enough to the substrate to cause that sort of issue. As a matter of fact, if you have a tank with enough flow you can get away without ever cleaning your sand. I went 6 months on an overstocked cichlid tank without ever siphoning the substrate and it was always sparkling clean.
Meanwhile, when you have gravel you will have to thoroughly and aggressively siphon the gunk and poo thqt settles inside it.