Jeremy,
I know you didn't ask, so tell me to get lost if you don't want this advice.
I don't know the exact measurements of your new frag tank so this is just a guess. It looks to be about 18"x18"x15". If this is accurate then the volume is approx. 21g. The weight of a 21g SW tank is approx. 168 lbs. That constant weight on what looks to be a piece of 3/4" plywood, that is only supported on two ends is going to bow in the center. If you can run one more 2x4 across the span, perpendicular (left to right in the pix), and in the center of the two existing 2x4's, you'll have a much more stable setup.
Keep in mind that the bowing effect will not happen over night. 3/4" plywood is decent for your application. However, over time, with the high humidity in that cabinet, the shelf the frag tank is on WILL DEFINITELY bow. Not 'if', but 'when' it bows the structural integrity of the frag tank itself will be compromised and potentially fail. This could take 6 months, a year, two years, or possibly longer, but it will happen the way it is set up right now.
Easy piece of cake fix as noted above will alleviate any concern about bowing.
BTW, that looks like it's going to be a real nice little setup. Congrats.
HTH
Russ