Goldfish with red streak on tail, and other goldfish spazing/twitching around and running into the bottom of tank!

[FishKeeper72]

Hello all,
Just recently (about 4 days ago), one of my Goldfish started to get these red vain type things on his tail. Then days later, all of my Goldfish except for my new one, started to do this spazing/twitching movement around the tank. They also have been diving to the sand as if they are scratching something off. They all seem to just do this when Im about to feed them or when it's time to wake up or go to bed.
I have 4 goldfish, 4 dojo loaches, 2 clown plecos.
My plan of attack: I plan on moving 3 of my biggest dojo loaches to a different tank. I have decided that my tank might be a little overstocked. Also the bigger loaches are annoying to the goldfish, so hopefully it will reduce stress for the goldies. I then plan on doing big water changes of 50%.
Do you guys have any sudjestions for my treatment plan? Is there something Im not seeing? Does someone have a different plan?

Tank size: 75 gal
Nitrate: 0
Nitrite: 0
PH: somewhere between 7.5-8.5
Ammonia: 0

 

[Colin_T]

The red lines in the tail are blood and its normally caused by poor water quality.

Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

If it's still a problem after a week of water changes, add some rock salt. Use 1-2 heaped tablespoons per 20 litres of water. Keep the salt in the tank for 2 weeks.