| Danzcool is right on.
Aged concrete and DIY applications are seldom cheaper than polyurea, in the long run. The need to apply the coating properly cause too many pond coatings to fail over the years. Failures occur from six months to ten years and then have to scraped clean and retreated in the problem spots. Polyurea is far better at sealing a pond that's developing cracks. The cracks won't stop and nothing stretches and flexes like polyurea to guarantee the cracks won't impact the seal.
There is an excellent comparison chart and discussion on all the coatings popular today posted on this board or on koiphen. It compares real costs of installation, reliability and other information that will certainly narrow down your choices for you and will help you to make your final decision in an informed way.
As far as harming you koi, if you're only dealing with a small pond, remove the fish until the job is done. On the other hand, if your pond is massive, remove the fish until the job is done.
It’s too much trouble to isolate the koi from harm while you're working on the pond and too easy for a mishap to be deadly.
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