Fin biting problems :/ And Shy Pleco, and dust from gravel.

[CandyHouze]

I just moved my 3 minnows into the 55 gallon tank with my 5 goldfish. One of the minnows, the biggest female recently given birth one, seems to enjoy biting the goldfishs' lovely flowing tail fins. They all have fantails and the beauty is being tarnished! Surprisingly, the largest one with the longest tail fin is getting the least of the beating.
This minnow is about 1/10 the size of all these goldfish but still has quite an additude. Is there any stopping this? If I feed that tank regularly will she stop? Or should I just accept this as a clashing relationship and separate them?

Next, the cute little plecostomus I have living with my betta fish (in a separate 10 gallon tank) has been very active until now. He used to vaccum all around the tank and poop more than I've ever seen a fish poop, but now he won't come out of the decorative pirate ship. That was always my betta's little safe haven and I think he won't go in it anymore since pleco is always in there :/
I think this has to do with the new gravel I recently put in the tank, about four days ago, I think it's throwing both of them off. My betta is sluggish and is leaving food behind which he usually doesn't do.

Even though I rinsed it out the gravel still had dust on it, so there's residue floating on top of the tank. I'm wondering it that is also what's affecting them. Can I safely turn on the little filter to clean it up, or will it stress my betta out to much?

 

[PzIkHo]

I'd recommend using a syphon to get as much of the solid particles out of the water as possible. Give it some time, and the fish will hopefully adjust.
As for the minnow, he may just be stressed due to an all new environment. I'd separate them from one another with some plexiglass or something, so they can get used to being in the same tank together, for a few days.