Guppy getting picked on

[Scinocca]

Hey everyone!
New fishkeeper here and I believe I am having an issue with my newly purchased guppy. I am running a 10 gallon tank with the aquaclear 20 filter and have been running it for ~ 3 months. My ammonia and nitrites are always at 0 and I do bi weekly water changes of 30%.
I previously had 3 neon tetras, a dwarf gourami, anda sunburst platy in the tank but a week and a half ago added a guppy as well. All was fine until 2 days ago. I saw that the platy was picking on the guppy such that it was no longer swimming around happily and instead hiding in the corner. Today o noticed it has developed a split tail (hopefully you can see in picture) and a yellowish line to this area I do not remember before. When I take the platy out the guppy happily swimsbthe entire tank again. I've tried rearranging the tank etc to no help. Does anyone have any suggestions. Does the yello yellow line there mean the guppy is sick? Will he heal? Is there any way to stop the fighting besides getting rid of one? Thank you guys for your help!

 

[Scinocca]

Didn't attach the picture properly. Here you go! Best I could do with him moving!

 

[Matt68005]

I recommend 30-50% water changes every week. Is the yellow line a sign of disease? no it looks like his natural coloration.
A small tank like a 10gallon should be taken down half way every week with water changing, larger tanks are the smaller changes that you hear about at pet stores and on the packaging.
He looks fine to me thus far.