I've made ginger bug before and it makes really good sodas, etc, but how do you expand the bug? I've look all over online and they show how to start and maintain a bug but not how to expand it. I'm guessing that I just basically redo the start procedures with the remaining ginger bug, adding some more water, sugar, and ginger.
Oxygen! The growth medium must be oxygenated from time to time. If one uses tap water then minerals and trace elements are rarely a problem but available nitrogen may be. The ginger beer SCOBY is quite good at using the ground ginger to obtain most nutrients other than sugars but sometimes a bit of yeast/wine nutrient helps. Friends have added a shot glass of apple juice once a week others have actually grown the bug in dilute sweet apple juice.
I usually add ginger and sugar, with a good shake/rousing, daily for (say) one week, leave it a few days for the ferment to subside and then decant the liquid from the ginger beer SCOBY and the ginger powder. The Demijohn I use is then refilled with tap water ensuring a lot of foam is produced to include oxygen in the solution. If done every week after a few months the SCOBY has usually increase (noticeably) in quantity/size.
It is worth using good organic (ground) ginger as perservatives may affect the SCOBY if present in any quantity. fresh/wet ginger can be used but it tends to float and can develop mould. I have seen boiled ginger mush being used (liquid filtered off into ginger beer brew)
"Every so often" - maybe once a month, I completely rinse the SCOBY from the build up of ginger powder. I tend to do that in a clean saucepan with a big but fine sieve. Pour the mix through the sieve into the saucepan then gently sway the sieve in the liquid so that the small grains stay in the sieve bu the ginger powder is washed through into the saucepan. After a while pour out the liquid from the pan and repeat the process with cold tap water. The point of using a sieve in a pan/bowl of water is that the gentle washing action keeps more fine SCOBY particles that can grow. If I just rinse the particles in a sieve under the tap then a lot of "small stuff" is lost. It is that small stuff that grows into the bigger white grains that are required.
Other than keeping equipment clean and free from build up of stuff (ginger, sugar, etc) I find that little other sanitation is needed. IMHO ginger beer SCOBY can hold its own against most competitors. I have been growing mine for over 6 years and I find it amazingly resilient if maintained regularly. I regularly have to give the excess to friends.
IMHO it is better to make up an "inoculation mix" with water/sugar/some ginger and leave it over the SCOBY for (say) 5 days in a demijohn. Then strain the liquid into bottles with whatever extra flavour/sugar or juice is required - rather than attempting to put the juice directly on the SCOBY. In such a way the SCOBY is kept in "optimal/clean/stable" conditions and the inoculation mix can make the natural soda as required by secondary fermentation in the bottle. Use PET bottles! I find that like wine natural sodas can really improve over time. They can be drunk after a few days but can improve greatly over a few months!