Fish cooks in a few short minutes - buy it fresh and cook it then and there. A slow cooker is, I think, the absolutely worst thing you would ever want to cook fish in !
Fish is a Fresh and NOW thing. You get it, you cook it, you eat it.
And we live on an ISLAND by the way so eating fish ought to be a habit...
Just whack it in a medium pan, a bit of butter, a touch of salt and pepper and Bob's your uncle. Lay it down and keep an eye on it, you can clearly see when it's cooked half through - flip him over and when it all looks the same (three/ four minutes max for a thin fillet) - you're good to go.
Mash it with a fork into a puree in a moment. Good baby food - no teeth required but home grown around our shores and so packed with protein and omega 3. Try tinned pilchards and sardines if you're unsure of fresh fish and don't want to cook.
All my kids had fresh cooked fish as their first solids and all love it to this day (13 years on). If you're not used to cooking it find a fishmonger rather than a supermarket and just be honest - just tell them you're interested and need advice. You'll make a friend, learn a lot and help your kids to a healthy future.
Never forget the Empire was built on FISH !
Slow cooker - no. More or less instant loveliness - yes.
You'll be banging out Mackerel baps before you know it girl...
And sardines on toast is (of course) ultimate nursery food for so many folk !
RECIPE: open a can of sardines into a bowl. (in oil or in tomato - doesn't matter - they're all from Cornwall). Mash with fork but leave some texture. Make toast. Spread sardines on toast. Re-Grill. Eat and grin. Grin again.
Case solved.
PS Sardines and Pilchards are EXACTLY the same thing. Same fish, same fish, same fish - never be put off by the label. Spanish name = Sardine. Cornish name = Pilchard.