Welcome to fishlore!!
How long have you had the tank? The cycle might just be getting started and the cloud you saw was probably whats called a bacteria bloom. The clarifier probably bound to all the proteins on the bacteria and they settled on the plants and died. Its possible the dead bacteria have now started feeding a mold, but I cant be sure without pics.
You mentioned the water reading were fine, what were they? Ammonia and Nitrite are only fine at 0
PPM, and nitrate less than 20. Alkalinity and pH arent a huge concern unless they fluctuate dramatically.
Lastly Ryukin goldfish each need about 10 gallons of water per fish as they grow quite large. The 8 might be overstocked and be causing all your problems. The issue with Goldfish is they produce a large amount of waste and can dirty water very quickly. I have two in a 20 gallon tank and it needs water changes sometimes twice a week.
If your new to the hobby there is a lot to learn, and its fun and frustrating at the same time. Use this forum and the links on the main page to learn as much as you can. The advise found at almost all petstore is bad and they are only in the hobby to make money. This should be your best source of information.
Feel free to send me a message or search the forums for answers. What I cant answer I will find someone or somewhere that can.
Again welcome!