majority of the bacteria are found colonising your filter, sand and liverock.
bacteria loss from WC is negligible.
also, introducing bacteria now and then will help to increase gene pool and add new strains, new stock. bacteria reproduce in two ways
- "sexually"
- and asexually.
the major reproduction of bacteria occurs asexually. where one cell clones and divides.
in "sexual" reproduction, bacteria don't actually engage in sexual intercouse. instead, they use a special organ called a pilus, to inject other bacteria and steal their DNA. this DNA may contain a gene that a bacteria lacks, and thus, it steals it and uses it. Once stolen, the gene is integrated into the thief's DNA and then it can use it to manufacture certain products, etc.
adding new bacteria culture can help to increase different strains and genes. which will enable the bacteria to integrate new genes and thus, creating better strains of bacteria.
it is not compulsory to add though. i personally never dose bacteria.