Lemon or Vinegar substitute for fish?

Tarragon. It has that tart, sour-ish flavor that vinegar and lemon juice imparts but it's a green herb. It's often served on fish. 

You can also try the dry lemon crystals or lemon flavored seasonings--they don't contain acids like the juice or slices do. I like the lemon pepper on fish. 

One classic condiment is a tartare sauce. I make it with mayo, sweet relish, and minced onion. Some use dill relish or add lemon. It's what I prefer.

Ketchup .

Either would be fine.

Get the flavor of lemon with lemon zest without the acidity that the juice has.

Add it to a bit of butter when you cook the fish. Add some green onion, a bit of ginger and some soy sauce. 

Or you could omit the lemon zest completely. 

Some fish does not need lemon at all.  A Piece of tuna cooked rare could be flavored with the same ginger, green onion and soy and a bit of wasabi or dry mustard if you want a bit of heat. Tuna can stand up to some pretty strong flavors. 

Other mild fish a bit of butter and seasonings that you like. 

You don't really replace an acid with anything except another acid. 

You just make a recipe that has a different flavor profile. Are tomatoes off the list - probably? Tomatoes and salt cured (not brined) olives are great with fish. Wine is always good. Pesto!

I was hoping I had a recipe for you, but all mine have lemon or orange ;(

Any citrus would work

Lemon vinegarette

Parsley or Dill sauce??