inconsistent plant growth behaviour between tanks

Anyone know what I;'m having the following problem, I have two new
Juwel tanks, a RIO 125 and a RIO 180. Both have been planted with
similar plants, eg some amazons, some grasses etc, both have some
bogwood and both have a single co2 generator - Hagen A-7690 - and I
have used plant food regularly with both. Both have 4in airstones at
the back of the tank.

However, one tanks is thriving (Rio180), so much so that I have had to
reduce the lighting period in order to cut down the growth and the
algae, while the other is only just about holding its own.

The only difference between the two tanks is that the rio125 is
overfiltered quite a lot (if's full of platies) with an extra fluval 3
plus, while the rio 180 which has mainly tetras has only got an extra
fluval 2 plus, as they don't crap so much.

Brian