sandbed/algae/nano issues - take your pick.

[rsisvixen]

Well in the tank world smaller is normally harder lol.
I would suggest not using your large tanks water for water changes in the nano as if you have higher phosphate lvls there it will translate to your smaller tank.
And the filter media will only remove so much of it as it will have a specific limit depending on the size and amount of the various media in it. If your lvls are really high it might be your filter reaches capacity in a few days and then won't absorb anything more, so you might need to replace weekly till the issues resolve

Always frustrating when you don't know why things are going pear shaped.