That probably won't hurt your plant, but there's lots of better ways to get good soil.

First of all, the fish would have to break down and decompose. If the anchovy is completely dry, it's probably not helping your plant much. Back in the day, we used to catch kivvers, chop them up and add the fish parts to water. I'm no expert, but I think that would be the way to do it. It stinks, but it's free.

Also, if you know someone with a good vegetable garden, ask to borrow a few shovelfuls of soil. Even if you can't find some nearby, decent soil doesn't cost that much - Just get a cheap commercial potting soil mix, you can always add nutrients later. From your post, I'm assuming you don't have much cash to put into this project. To get a decent plant, you will need to spend some $$$, but not a lot.

Tell us the details and we may be able to help you more. For example, what kind of seeds are they, is this a closet grow, what kind of lights are you using, etc, etc? Also, do some research on your own too - most of the questions you might want to ask have already been answered here somewhere. Start with the grow info on the main page of cannabis.com or use the Search feature on the forums.