Bare bottom vs sand

I am about to get 3 senegal polypterus for a 4ft tank and was wondering what do you guys have as a substrate and why.

i currently have a 30g bare bottom community tank with lots of caves made out of bogwood pieces and plants potted in small jars. Do you think that the bichirs would like environment like this? Or should I leave the sand as it is (20mm just to cover the bottom) plus the same decor as in 30g.

i don't need a substrate to act as a bio filter as I'm building a whole sump for that haha plus im doing weekly water changes and clean up on the 30g so 30min for the 4ft tank won't make a huge diffrence I am interested in polypterus behaviour more than anything

the 4th is a 66g (i posted on this forum that i have a 55g tank but in fact it's 55 UK gallons. I didn't realise that uk gallons are different from us when I was doing the convention) I have 2 more tanks - 30g - that's the one with bare bottom, and 25g. and spare 20g one that i use as quarantine or hospital tank. I think that bare bottom tanks look very clean, the way i've done mine is to place lots of deco and white styrofoam below tank. This brightens the tank ALOT.

So you you think that a thin layer of sand can do? the sand I got with the fish tank is very fine and I'm afraid of anaerobic bacteria forming if its too deep. plus finding a cheap (affordable) aquarium sand where I live its like trying to buy an elephant!