I guess I will start on this second round..... keeping koi for about 6 years now and I try to remember some of the mistake that I made.
#1 – First year, I started up with 125 gal plastic pond from Lowe's, added rock in the pond and kept 6 koi (10” to 14”) in it. Not for long they started to have issue since I couldn't keep up with the water change, lost a few. But didn't stop me to bring home more fish.....
#2 – First cold month, two of the fish were getting sick didn't know what to treat so I moved them in the garage to bring up the water temperature and to add salt. It was late at night so I used warm water from the house for a quick change in temperature, only found out in a few hours later that they didn't make it b/c they were shocked.
#3 – Another mistake when I moved a fish into the garage for treatment, I was in a hurry to go out for dinner with some friends and forgot to put a cover on the tank. When I got back few hours later I found the fish on the floor.
#4 – After I got my first pond build, I didn't want to move my bigger fish in to the new pond yet so I went and bought 4 small koi to test the water and system. Well, three of them were slowly disappeared and nowhere to be found for days. A week went by, when I cleaned the pump basket and found them all there. 1st pond don't have bottom drain, just have the above ground pump with 2” pipe extend to the bottom.
#5 – With the first pond, I hired a pond guy to help with laying the rock around the pond. When I got home from work I was upset b/c all the rock with sharp edge were pointed into the pond. Didn't have time to fix this issue right the way so I let it go for few months and when the fish spawn they cut themselves, thank God no one was infected.
#6 – One time I did a PP treatment and didn't add extra air pump, I almost kill the fish since two of them started to roll over on their side. Didn't use PP again since.
#7 – Had the water pump and air pump hooked up in a same circuit and when the circuit break went out (don't know how long) I found all my fish struggling for air when I got home from work. I have them plug into two different circuit now.
#8 – Doing water change one day on a 600 qt with submersible pump and forgot to turn off the release valve when I had a surprise visit from my neighbor. When the visit over, I realized I was in the middle of water change and the valve still on, ran out side and the qt was empty... fish were laying around but were still breathing. I felt horrible.
Those are what I remember at this moment....
After 6 years in this hobby, I still make mistake but here to learn …..