Its under your tank stand as a shim remember:razz:

I have 2" thick ply wood floors with thinset, steel mesh, more thinset, and then marble/porcelain tile on top. 300 gallon fish tank with stand will weigh 4000 lbs.

Under the floor are 2 I beam joists 16" apart from centre or 12" apart from edge to edge supporting the 7 ft tank length wise. I boxed in the I-beams and put 4 steel teleposts under the joists. One side of the joist rests on a concrete exterior wall. The tank is 12" from the exterior wall. The joists are 16' long and the other side sits on a steel beam.

So far with 1500 lbs or so on top of the joists right now there is 0 degrees of deflection and that is without the teleposts installed. I would be surprised if I saw 1 degree of deflection.

I think it depends on material as well could be 5/8 for ply but should be thicker for OSB and could be some weird number like 23/32" which is basically 3/4". Also joist span is between 16-24" typically so that can play a role as well.

two layers of 19/32" glued and screwed on 6" centers. will give you the min code of L360 deflection, if it is on properly sized joists on 16" centers. problem is so many builders cut corners and the L360 is a min code which in many cases is not sufficient for natural stone or larger tiles (these can require up to L480)

with the building boom over the last 15 years there are a lot of bad builders that would not put the second layer of plywood, or neglect to glue the two layers of plywood togeather, or spaced the screws every 8". no glue and a larger screw spacing can actualy add up to a lot of money in a large tiled area. you figure the recomended aplication of the glue is two tubes per sheet. at 12 bucks a tube combined with less screws you can save over 100 bucks in a 16x16 room. make this a subdivision with 40 houses, and other cost saving measures it can add up to a lot for a developer.

if you realy have a 1.25 or 1.5" subfloor, with ceramic tiles you would be fine.. the only thing I would recomend is adding extra blocking between the joists under the area to help spread the weight over more joists, which also stiffens a floor.

Steve