Do I need to reinforce the floor?

[kamil]

How large a tank can I get safely?

The house is from 1970.

The tank is to go perpendicular to 2"x8"joists which are 16" apart.
Parallel to the tank, (in the wall) is a beam composed of 4 2"x10"
Under one corner (of were the tanks is to be) there is a telescoping
support under the beam.

This seams strong, but ..
The 20 + 15 gallon tanks I have on a 4 legged metal 3/4" square tube stand ripple when a person or dog walks by.

Cheers,

Kamil