A skimmer is really handy to have and makes less work for you in the long run.
Filters can't remove ammonia or nitrites or nitrates from the water per se. They create a bed for bacteria to live and the water flows through this bacteria bed. The ammonia is converted to nitrites and then bacteria consume this which gets turned into nitrates. Filter media also traps debris and more and more of this is turned into nitrates. Nitrates don't get removed unless you do it and high nitrate levels lead to dead inverts. With the media the debris gets trapped in the lr
and sand and the critters eat it up keeping the nitrate levels lower (if I am getting this correct, hopefully I am). The LR
do all of the conversion of ammonia and nitrites so there is no need for even having a filter. Water current is important though so your filter can easily be used for that purpose.
With a DSB
(sand) you can't have a tube in the bottom of the tank. That sounds like a modified UGF
of some sort. You might be needing to get some powerheads for extra water movement as you need to create of current that moves 10x the tank volumn per hour or so.