i have a 55 gallon tank, artifical plants, and a home made background i saw on youtube where you glue lava rocks to gutters and put it inside the tank, looks amazing.
i HAD two koi fish and 4 big gold fish and they died, i dont know why, i tested the water everything came back safe, i did a 100% water change and let it cycle before adding more fish, i bought 6 more gold fish and they seemed like they was doing okay, so i got a koi fish, within 48 hours all 6 gold fish died, and now my koi is swimming at the bottem,(the same thing the gold fish did before they died) what could be killing them?? should i do a 20% water change??

also with my air bubble rock(more like an air bubble strip) is producing the bubbles like they should but they are not breaking at the surface once produced, they are spreading out across the tank, some of them slowly popping, some floating towards the filter and getting pushed under water then back up. is that okay?

Koi really aren't meant for tanks, they grow way to fast and are super messy. Comet goldfish also aren't the best choice for tanks either

Fancy goldfish would be a better choice :) and it works 20gal for the 1st goldfish and then 10gal for every addition one, so with a 55gal you can have 4 goldfish (a push to an absolutely maximum of 5 if you are prepared to do extra work). I personally have 1 large fantail that has a 50gal tank all to itself!

When you tested the water, can you give the results here please :) What filtration are you running?

Also when you say you cycled the tank, how long and with what??

To me it, at a glance with what you have written, seems you are overstocking an under cycled tank, this generally gives the results you have gotten :)