South American theme sounds good, are we going to see any pics though?
I have hatchet fish and would like more, mine are marble hatchets and I lost quite a few initially but the remaining ones are doing great last I saw off them through all the vallis. I will have to do a head count. Silver hatchets are supposed to be hardier, can't remember where I read that.
Are these the fire eels you mean? This info suggests a 400l tank. I don't know much about them but sound interesting.
http://www.aquahobby.com/gallery/e_Mastacembelus_erythrotaenia.php
So what exactly will be the fish list for your 300l what you already know you are adding and what you ideally would like to add but put it as a list. Maybe with maximum potential size next to each one and how many of them each.
What type of plec do you plan to breed in the 125l?
What decor in the 300l and equipment, lighting , setup/outlay etc will you have?