Structural engineer here, I recommend you look up plywood span tables. Lumber manufacturers publish tables that tell you the load carrying capacity of a sheet of plywood based on the thickness and the span (the distance between supports).
Example is Page 4 of this document: http://www.pacificwoodlaminates.com/...SpanTables.pdf
Essentially, the tables published will give you a maximum weight per square foot that the plywood is rated for depending on how far you space the supports.
A 40 breeder is 40 gallons (roughly 340 lbs) over a 36inx18in area (about 4.5 sqft) which means you're looking at a psf value of 76psf.
You need to look up the available plywood type that you have (in terms of grain pattern) and from that you can decide if the 1/4" plywood is capable of carrying the 76 psf based on the minimum dimension you have between supporting members.
You also want to go L/360, as that is deflection that will occur in the panel over that span, since you don't want the bottom of your tank bowing, L/360 is the best criteria for you.
1/4" plywood isn't really a structural plywood, so not much data is published. But if you were to go a 3/8" sheet for example, you'd need supports at every 16" at a minimum (which can take 79 psf before it becomes worrisome with regards to having too much bending).