It *looks* reasonably sturdy, but I am not sure if it is designed for 700+ lbs.
A lot depends on how it is constructed. Solid wood vs Particle board, how it is jointed, etc.
If this is one of those assemble it yourself units, I would not trust it at all. Those are designed with edge jointed hardware held together with blind screws. One of the weaker ways of joining boards. Any loosening of the screws can mean disaster. I wouldn't use it for any more than a 30 gal
or so. <That is based on weight only .... there is water involved as well .... If it is not solid wood, water/humidity will also loosen screws & swell plywood/particle boards/MDF .... again a risky proposition.>
So, for 55 gal
(or any big tank), I would go for a specially designed stand. <And even for commercial stands, I'd look carefully at its construction ... avoild all MDF & particle board ... some of the cheap Wally world stands has exposed particle board edges & are doomed to fail in a few years ....>
If you are an accomplished DIY'er or a cabinet maker, you can make something like that work ... It means reinforcing/redesigning all the joints, putting in supports, make sure it is level & cannot rack, etc .....