i got one about 3" for fifty euro at my lfs about three weeks ago( a customer had done a swap) probably worth a bit more cause it was already accilmitized
I bought mine at 1.5" for £20. The little guy is in my 125 litre tank. Some GN grow bigger than 8" and when they get over this they shouldn't be in a 126 litre tank. The GN that grows to 7"-8" will be fine in your tank.
There are (I think) 3 different species with the common name "Gold Nugget". One is the L081, another is L018, and the last is L177. They vary in size as they are completely different species - but I *think* L081 is the smallest. How many gallons is 126 litres?
Hmmm if it was a plec only tank it might make a difference, but personally I wouldn't have anything that big in 30g - and to be honest Gold Nuggets have a poor reputation for being extremely sensitive on arrival in a new tank, so the more stable the water the better. There are lots of pretty smaller plecs though