Nimbus,
My GSP's are about 5" so nearly there, this is the rule to live by when it comes to GSP's, keep GSPs at low-end BW when juvenile <2†at SG 1.005-08 at 2-4†medium BW SG 1.010-15 and adult 4†SW SG 1.018-22.
As far as the algae goes, you could look at putting a UV sterilizer on there or making sure you clear uneaten food as that possibly is causing the nitrates to sky rocket (I am assuming you have checked the levels) which will be making it a prime place for algae, out of interest what diet do your puffers have.
Hope this helps.
Opius