Hi noah first of all,great work on starting to build stand for your tank.often when we get started on anything in life,the rest falls in place as we go,but having a well laid plan,research is grrat to have and modify of needed as we go.
Anyway your stand them extra bits on top you just added on after im guessing,if had extra sticking out at bottom you could run some wood vertical from bottom to top like in my screenshot below, but because the base of the stand is the most important ( where the tank actually sits on) its more important than not having the tank level ( ovbiously needs to be as well levelled as we can achieve,otherwise side panels of glass can twist and shift creating a leak or worse still a crack/rupture)
It looks like the top pieces of wood you used are not all totally flat with each other and put a tank on this and will cause stress points ( basically parts of the wood sticking up higher on some parts and most the weight on that part with downward force and all that weight of water as 1 litre water = 1kg in weight.my qt tank is 54 litres and 15 us gallons and weight of water 54 kg then weight of tank and salt water weighs slightly more so your 20 us gallons roughly about 72 kg just the water alone.and remember one piece of wood providing it true and straight is just straighter than several pieces of wood screwed or glued together etc.
Me personally i would be removing them outer 4 pieces of wood at top and adding some longer pieces so didnt have them bits wood that looks like are sagging over as i would be worried about the stress points and creating problems.and adding some vertical pieces of wood from bottom- top of stand in all 4 outer edge corners as in screenshot.
Unless all the top of stand is true and straight using spirit levels and no pieces of wood protude higher than another piece and maybe the width of your tank from left to right only goes along that clean bit of wood at front of top of stand and will not sit on them 2 edge bits that looks like they sagging down.but my end stand would need to have perfect flat base/ be level best i could and be able to hold the weight .
But one thing im pretty certain on,you have done a better job than i would of done when im 14 years old so good luck buddy ^_^