Arapaima grow huge, what is your plan?
Get some good test kits as you need to keep a eye on your water quality, a 400 gal tank with a ray is a large challenge, there is a lot to learn if you are asking questions about cloudy water on a cycling tank.....please, please, please keep us updated, be honest and take advice as you will need to learn a lot of what not to do quite quickly..... Good luck and we are here to help.