Yeah, tightening up is your mistake. And it's not really your fault, it's a general misconception that has even made it's way into books and guides, etc. When people are playing a very wide 
range, you need to loosen up. If you are playing 80% of hands, then I can profitably play 60% or more of hands when I have 
position. Assume we are better postflop, and we will 
rack up against the 
fish playing 80% of hands. We just need to find out his tendencies and tailor our ranges to 
exploit that. 
For instance, if he is playing 80% of hands, so we decide to play say 50% of hands (other factors such as players left to 
act, 
position, etc needs to be taken into account), and we figure out that he just bets 3 streets with a very high frequency. In that 
case, that means his betting 
range has loads of bluffs/
air/weak hands in it. Which means, with our wide 
range, we should start calling with a wider 
range of hands. 
Say he calls with a wide 
range, then we need to shrink the 
range of hands we 
bluff with (because he doesn't 
fold), and we need to widen the 
range of hands we 
value bet with (because he calls with worse so often). So in this 
case we can 
value bet his with a lot of hands, hands that would normally be considered '
marginal'.