He is actually still probably right.
If we say the fish is 2cm wide and 7.5cm long (average length according to wiki) That's 23cm^3 Assume the fish is neutrally bouyant so it's 1g/cm^3(I know deep sea pressures change density, but let's just go with this)
1 quadrillion*23g= 23 trillion kg
So like... 1/32 of that is still around 100x bigger than the trees.
That seems dubious, trophically. Whatever the prolific bristlemouth species eats would then likely be the greater biomass species