with 1.5" thick subfloor and 16" joists span a 13" wide tank simply won't cause enough deflection to damage the tile. Also that load rating I mentioned is based on deflection not failure.

Here's a good read. Actually came upon this through a link on another thread dealing with tank volume.

http://badmanstropicalfish.com/articles/article28.html

I can't see the subfloor being 1.5" thick...where are you able to see the subfloor Coleus? If it is indeed 1.5" thick you don't need to do anything to the tile. It won't deflect enough to dislodge the tiles or crack the grout.

As far as the actual weight of the tank, I told you from the get go that you don't need to support the floor. Thankfully some other people have come in here and supported this as well. ;)

Myka, I believe subfloors are required to be 1.25" thick for tile, typical subfloors are 3/4" so when tile is to be set builders lay an extra sheet of 1/2" on top. However depending on the builder extra 3/4" may be more available so they may double up on the 3/4" to make 1.5". I've seen this a few times before so I wouldn't doubt it, but even 1.25" is plenty.

as true as it is not many people use the required thickness for subfloors(5/8" or 1 1/4 including the original ply) ) most just use 3/8" to match with hard wood easily by the time tile goes on........

well the stair to my basement was not finished so i can see that there is a .5" layer on top of 1 inch subfloor. Then my tile is on top of that. So I assume it is 1.5" subfloor that you guys are talking about. I better take some pictures and post it here. I will get some engineer guy come over and take a look to make sure.

Thanks for everyone inputs here, we should really have a "Thanks" system going on this board.

ok i guess my subfloor is 1.25" not 1.5. Damn cheap builder lol

That is floor level, not a stair, right? I think you're good to go. The worst thing that could happen is the grout cracks or a tile pops up. The tiles themselves shouldn't crack unless they are el cheapo. If the grout cracks or a tile pops up use silicone-based sanded grout to fix it up. It is pretty easy to work with. It is what is recommended in showers and baths for in the corners and between the tub and the tile. It works well and comes in the same array of colors as the grout is. Made to match (although the match isn't perfect).

Mapei is the brand I'm used to working with. Now that I read the manufacturer website it is actually acrylic-based caulking, not silicone.

Maybe I'm not remembering properly, but I thought standard subflooring is 5/8"? Then builders add 1/2" for tile which technically makes 1 1/8", but most just call it 1". It's been awhile since I was in the industry.

Newer homes should be 3/4" as far as I know, mine is.

Could be a BC/AB difference too. I should dig out my BC Building Code book...wonder where it is.