Rack Shelf Support Advice, Please

[demicent]

I did try searching the topic on this forum, but for some reason once the results come up I am unable to scroll down to view anything past the first 3 discussions or so.

I just put together this extremely yellow  Dewalt rack Model #DXST4500 , which has shelves about 44" x 18" made out of some composite material that is less than 1/2" thick.  There are three supports under each shelf, one each a couple of inches from the end of the shelf, and one in the center, which leaves about 18" of unsupported space between them. The other two straps provide stability, but not load support for the shelves.  Photo attached.

I plan to use these shelves for rimmed aquariums,  1 - 20 Gallon Tall, a couple of 10 Gallons, 1- 5.5 Gallon, and some 2.5 Wabi Kusa and Hospital tanks.  

My question is, of course,  is this enough support for the filled boxes?  The 20 will straddle 2 supports but the rest will not.  What are the alternatives?  Lay plywood over the shelves?  If so, how thick?  Lay 2x4's over the shelves?  Lengthwise or across the short side?  Would smaller lumber work, like 1x2?  

Any advice is sincerely appreciated.

[lefty o]

i would search dewalt for that model, and see if they show a load rating for it. 

[demicent]

"The three black laminate wood composite decks can each support an evenly distributed 1,500 lb. for a total of 4,500 lbs. " but that does nothing to prevent the composite shelves from sagging with time.  That's my fear.  Especially if they get wet, which they will.

[MickS77]

I'd replace the shelves with 3/4" plywood sheathing. Overkill won't hurt anything and you could support larger tanks in the future.

[demicent]

I would want to paint the plywood black.  Should some sort of varnish or water repellant be applied, I wonder?  Also, I can't trust the local big box store to cut the wood to the precise correct measurements so that it would fit inside the rim, so I would probably end up laying it on top of the current shelves.  

Edited January 5, 2021 by demicent
Forgot something.

[lefty o]

a couple good coats of paint will help retard water. at least good enough that if you wipe it off in a reasonable amount of time, it wont hurt the plywood.

[StevesFishTanks]

Are you able to cut wood? They sell project panel sizes that are 2x4. Can be trimmed to the size of the shelf needed. You can use the dewalt insert as a template. May be the cheapest option if doing just one shelf.

https://www.lowes.com/pd/3-4-in-Lauan-Plywood-Application-as-2-x-4/1000068905 

I use spar urethane or polyurethane to seal. 

[demicent]

I was able to order that plywood online at Lowe's as a 4x4 sheet, and put cut instructions on the order. That will give me two shelves.  My lower shelf will just be for the power supply.   I have a saw but I'm pretty bad at it.  If needed, I know one of my neighbors will trim it for me.  I ordered a black paint, so I should be good to go.  I thank everyone for the help and ideas!