Hi Georgina, good to hear you have been trouble free for a good while.
In my humble opinion I'd say your Plec was rapidly reaching the point
where the tank is going to be too small :-( Time to get bigger I'm
afraid, I'd go for a 4 foot one if possible, as I fear your Plec may
well reach 18 inches in time.... even if it only gets to 12 inches or
so it's going to need some room. If you go for anything smaller you
might find yourself running into the same problem in a year or two.
Anyway hope this helps, sorry for non-metric sizes but I'm of that age
lol
Regards
Steve