Oven baked cod fillets get soaked in their own liquid I baked cod fillets twice. The first batch was fine, the second had the fish soaked in its own liquid.
(Two different batches from the same shop, salted 1.5 hour before cooking, within expiration date).

I used a basic recipe:

cod fillets
olive oil
garlic
parsley
salt, pepper


Baked in pre-heated oven @ 180C/fanned air for 10-15 minutes

How can I prevent this loss of water?
As the cod fillets were two different batches, chances are you just where unlucky with the second batch. Even within expiry-date, the amount of water food loses while cooking can vary vastly with age and quality. And even good fish shops can have watery batches at times.  

To generally avoid this soaking happening, I would rest the cod on a cooking grate or - this is a personal preference - on some pre-cooked potatoes.  

A small factor may be oversalting, but chances are it is actually the cod.