Why?? It's a good way to spend your discretionary income. LOL Seriously, you don't need to. Once the bacteria are established the only time you would need to boost them would be when you kill them. For instance using chlorinated water to clean your filters. Aquascape, and other pond product suppliers want you to think you have to do this so they can sell you their stuff.

Are you having issues w/ water parameters? Just curious why you are purchasing bacteria to add.

By the way, I hate to be the bearer of bad news, but your pond 125 gallon pond isn't big enough for even ONE of those koi to live in, let alone share it w/ 4 other koi and 5 comets. :(

I didn't even add bacteria starter, much less add more. I have a small biofalls and leave that alone, a pondmaster filter with both mechanical and some chemical (charcoal filter pad) filtration.

I add it in the spring when I first turn on my bio-filter then usually don't add it again unless I feel like it. I don't know if it reallly needs it, but I figure it won't hurt. I use the Microb Lift brand. I don't know if it matters what you use.

When I clean my pond and filters I always reserve a piece of the quilt batting I use as a filter. If it will be out of the flow for an hour or more which is not common I put it in a bucket of water with a bubbler under the batting. That prevents the bacteria from dying. Once I am ready to put in the new batting, I just put the piece of old batting in first. It acts as a starter.

If I didn't change out the water when I clean the filter, even that would be overkill. There are plenty of bacteria and microbes living in the water and on every surface in the pond. They reproduce fast. Turning off the water flow turns off the oxygen that allows aerobic bacteria to stay alive. Water that is still loses oxygen and anaerobic bacteria, the kind you don't want, thrives.

I am posting a site you may find interesting.

Here is a link that might be useful: Ron's Pond Scum

We have a couple of five gallon buckets that we fill with pond water
and then clean the filter material in these buckets by rinsing then up
and down. This water is then carried around the pond and garden
and used to fertilize the flowers and plants.
We never use the hose to clean the filter material.

Etrn, as everyone suggested no need to keep adding bacteria in an
established pond.
As Admink said, your pond is not big enough for koi...Better start
thinking about a bigger pond.