Been on vacation for 3 weeks how much water should I change?

[KingAh123]

My tank is a 375-liter tank.
Livestock:
2 Ocellaris clowns
1 bicolor blenny
1 Yellow watchman goby
1 Diamond watchman goby
1 Green clown goby
4 blue-green chromis
1 Blue sapphire damsel
1 Royal gramma

They are all very young fish the biggest one being the blenny who is about 7-9cm I have only had the fish for a year.
I have a small colony of Ricordea Florida which is the only coral I have currently.
Did a 20% water change before I left my uncle fed my fish while I was gone at the amount I told him to feed.
How big of a water change should I do?

 

[PR_nano]

I would test nutrients and decide after that. Might be able to resume normal routine or might need bigger water change.