I have about 6 slaughter fish already caught and I can seem to find or see the next one. Is there anyway of fixing this or do I need to start over again?

You could use the Detect Life spell. Sometimes the fish get stuck in the rocks. Happened to me.

It's been awhile on that quest, but if I recall, when you have it set as your active quest, it will give you a green arrow to the next fish.

Is there any reason why I wouldn't run into Slaughter Fish any other time than when I do this quest? Are they just not in game?

There are other Slaughterfish in the game but for the quest they have to be Rumare Slaughterfish.

...I believe thats the quest where the guy across from the Wawnett Inn asks you to find him a certain amount of slaughterfish scales, and in return he gives you the Jewel of Rumare ring (which has the constant effect underwater breathing)...you do in fact have to make it your active quest, and the green arrow will guide you to them...jump in at the pier behind the wawnett inn and go clockwise throughout the surrounding Imperial City area...dive down and look up, you'll have 'em all in about 10 mins or so. Bring an elven shortsword. Besides that, you will run into them in the strangest of places...they'll surprise you like the nirnroot!

Yeah, I had trouble with this quest too. Detect Life is the best way around it, in my opinion.

...check out the lower Nibany Bay area by the fishermens camp at night, and while you're at it snatch up some nirnroots along the bank!

I've noticed Rumare Slaughterfish are no different from their ordinary cousins and can sometimes be found flopping around on dry land. If the green quest arrow is pointing somewhere impossible, it may still be correct!

More than once I've actually had the green arrow get stuck on a fish that I'd already killed, making it very difficult to find the next one. Now I don't start that quest without saving first.