Eating fish is VERY healthy for you - and it tastes great with tartar sauce! Luckily, tartar sauce is a naturally low carb concoction. It is primarily mayonnaise with flavorings.
2 cups mayonnaise
2oz chopped dill pickles
1oz chopped onions
1 Tbsp parsley
Chop up all of the pickles and onions, draining away the liquid. Then mix together the various items. You can add salt and pepper to taste, if you like.
Tartar sauce is best served chilled, so if you began with warm mayo (from a new jar) put it in the fridge for a while to chill it down to a cooler temperature. When serving, it works well to get little glass ramekins so each person has their own small bowl of it to use based on their personal taste. This also lets them add more salt or pepper to their own.
Mayonnaise has 0.3g of carbs per Tablespoon. You get pretty much zero carbs for the small amount you'll have in a given serving, and the same is true for the parsley. So really, if you determine how much of the tartar sauce you ingest by Tablespoon, just multiply that by 0.3g. Some people use a ton of tartar sauce and others use only a little, so it really can vary wildly person by person.
Note that one thing to do is to smear a small layer of tartar sauce over your food, rather than making big globs of it. That way you get the nice flavors of it without ingesting a lot of it without reason.
Ideas for personalizing:
capers - they are tasty and cute too!
lemon juice - adds a nice, zesty tang
sweet relish - a good substitute instead of pickles
horseradish sauce - another idea for a nice tang
garlic powder - in case any vampires live nearby!
olives - I like olives in everything
lime juice - for those who prefer their juices green
Pretty much none of these will really affect the carb count. Again, the amount you end up with of the little 'add ons' at a 1oz or 2oz rate is minimal. And since you tend to use tartar sauce on a zero carb fish dish, the end result is delicious and very good for you!