100 litres is basically 25 US Gals.
I think 100 litre is the
minimum for
one fantail goldfish
" Tank size is the biggest mistake that newcomers to goldfish make. Fancy goldfish need a tank that is a
minimum of 100l for
one fish, with an additional 50l for each additional fish (so, 150l for two fish, 200l for three and so on). While they
can be started off in smaller tanks, getting a larger one should be made a matter of priority, as goldfish are fast growers in their first few years. "
Quoted from :
http
/www.fishforums.net/index.php?/topic/417799-goldfish-for-beginners/
Well worth reading up on that article for some goldfish information.
But, 100 litres is a fairly good size of tank for a shoal of WCMM, I like these little temperate / coldwater fish myself and have considered getting some of these guys many times
More info on WCMM :
http
/www.seriouslyfish.com/species/tanichthys-albonubes/
Not sure which flagtails you speak of, Florida Flagfish are what springs to my mind as these are fine in coldwater set ups (18 - 30C or 64 - 86F) and will be fine in a 100 litre tank for a pair.
More info on Florida Flagfish :
http
/www.seriouslyfish.com/species/jordanella-floridae/
(if you are thinking of Flagtails, I must mention certainly not Flagtail Prochilodus as these are far too large for a 100 litre tank)