Fish Tank Weight and Reinforcing the Floor?

[Hal]

I am looking st starting my first saltwater tank. I really want 120-150 gallon tank. My question is that I have 2x8 floor joists 12 inches on center. I have 3/4 inch plywood for my sub floor and the weight is within 12-18 inches of a load bearing wall. If I get a 6 foot tank I would be on at least 5 joists holding a bunch of weight. How concerned should I be about the floor failing. Or would I be ok? Any help would be great.

My gut check says you would likely be ok. The fact that your tank will be on 5 joists is pretty much what I'm going on. You're probably averaging 425lbs per joist. However, to give you a better answer, we need to know the dimensions of your tank and the total length of the joists. This is the first time I've heard of 2x8 joists. The strength of a joist is going to be a function of its height (for solid boards), or the heights of the top and bottom portions of manufactured joists (in an "I" shape).

I recently strengthened my floor joists by screwing and gluing a 3/4 plywood to the bottom (and connecting) 3 joists. This is on the bottom of a typical manufactured joist (I can't remember the dimensions of the joist). I calculated that this extra 3/4 inch increased my load bearing by about 20%. This was a variation on a "strongback" support. Look it up.

The other thing that most people don't take into account is the shear pressure on the plywood flooring. Usually it's not a problem, but with extra big tanks that only have a few legs for the stand it can start to become a concern. This is easily solved, however, by distributing the load over a larger area with a metal plate (6x6, 8x8, etc.), or, even better, by using a piece of angle iron that extends between 2 legs. (Unlike a plate, angle iron is very resistant to bending.) For a 150, though, I wouldn't worry.