I think the floor is 3/4" plywood... mine is.
The one's in my unit use metal screws through floor and through a sheet metal plate (not that thick), but they hold well. But, the backside is accessible.
My old Greywolf had no backing, but relied on adhesive and just wood screws. I questioned how good this would be, until I took them off... and found out they would have been okay.
If you want a tie point above a tank and don't want to drop it, I would use these (attached) rather than just wood screws. Probably would go with the 5/16"... these really, really hold. I've used them on several wood project and can vouch for them. The one for soft wood is what you need. Of course, you would have to have to use a 5/16" machine screw threaded to the head at the proper length and head type (probably flat). I would also cut the recess hole tight around the hold down and seat it into a bed of silicon adhesive, as well.
Hex Drive Threaded Inserts-Choose size - Rockler Woodworking Tools