(1) It is pine. If you are going to finish it, it HAS to be painted. Pine, because of the sap content of the wood, will never take stain evenly. No matter what you do it will look like ass if you try to stain it.
(2) In additional to it being pine, it is rough grade (knots, sapwood, bark). It is not going to be dimensionally stable. Plan for the fact that it will have a strong desire to warp, bend, or cup.
(3) No matter what you are using this construction for, it needs diagonal support. All its strength right now is vertical. The slightest weight on the top and a sideways push, and the whole thing will collapse sideways.
(4) Easiest solution, skin with plywood, seal, sand, paint. If you are going to use it as an aquarium stand, consider a clear epoxy sealant (at the minimum) or better yet - epoxy sealer, sand, and then epoxy paint. Your stand will look better than 95% of the handmade junk out there