Hi Maroder
I make a thread on the fishing mod you provided me with via a link. Turns out the inserter has some trouble picking up the correct fish. I asked the mod creator about it and he said the following:
Can you ask by the hsc2 the new types of catched fishes? The inserter supports filtering and if the result of the mined fish has another name, then you can filter it too.
For example the entity "small-fish" on mining gives "small-fish" item and the filter in the insetter will support it.

Here is a link to the discussion: https://mods.factorio.com/mod/FishCatcher/discussion/5e176891dcb7d6000deee070
Thank you

This mod does not filter on fish. It allows you to catch fish from a reservoir, which is happening now without modifications. More precisely: the mod turns the fish into an item, provided that from the fish an item is obtained that matches the filter. Another fish is caught “without filter”, provided that the inserter works (only a factorio developer can change this).

So if I'm understanding you correctly, there is no way for me to catch the grown fish automatically unless the factorio developers implement this?

There are several options. The simplest: sometimes use a deconstruction plan with a filter of the desired fish on the pond. Or use some kind of mod that will do this automatically.
As an option without using robots: turn on inserts when the fish is older.

Yea, I also figured that bots + deconstruction plan would be the easiest. Till I get there, i'll do it manually. Thank you