Pleco and frog both died not sure why.

[Goes to 11!]

+1 to double the filtration. Another + 1 to QT

why does it say it has a max of an 80 gallon tank.
mine is alittle under 50.

The short version is that the real world effectiveness of your filter is roughly half of what it says on the box, And here's why:

Generally speaking, A filters flow rate is tested with no media in them. This is obviously not how a filter is run in the real world so when you add media the flow is limited.
When that media gets gunked up as is par for the course [But that's also when it is effective] the filters actual flow capacity retreats even farther from the laboratory conditions that the print copy on the box claims.
Make sense?

Floss is good Bio-max would be better IMO.

I repeat:

'Messed up', What does that mean?

Any major parameter differences between the shops and you? Can you detail your acclimation process for us?