Why do some of my guppies get long and thin?
They live in a heavily stocked tank with other guppies, glass catfish, and heaps of bristlenose. They are all home bred. I have seen TB, or consumption, in White Clouds, and this doesn't look like it. They don't have the same hollow wasting look, they just don't seem to put on any weight. They are not being bullied, or missing out on food - 'cause I overfeed stacks. Occassionally I see my guppies scratching on the gravel or plants, but it isn't all the time. And no, I don't use test kits. I use well seasoned water that I condition properly. The PH would be slightly above neutral, and the water is boiled, so soft.
Should I start with some salt?

guppies do better in hard water. have they started looking this recently or were they always like that?

you're better off using aged tap water than boiled cause boiling takes out some minerals out of the water.

I feel like they were just born that way - 'cause it is only some of them, and it isn't like they have wasted away. They just haven't ever put the weight on ...

it's probably the genetics or sth then

Yeah ... I'll just keep takin' 'em out of the gene pool!

Without knowing the history of your guppies, it is hard to determine if it is a genetic deficiency or strait that causes this. Some guppy fry can be born without the necessary supplementation as other fry; this is especially the case in damaged or sick females, when giving birth. There are many ways to combat this. Firstly introduce new blood, that way you’re adding genetic diversity into your gene pool, which can add, finiage, body shape and colour as well as higher and better fertility etc. Selective breeding won’t help much if the female is still producing deformed fry, if it is a genetic trait the fry will carry it as well. So at the very least I would recommend you finding some new blood, and knocking off the female.

Cheers Shae

guppies...as similar as guppies are..they do come in heaps of diff sizes...it actually made me laugh when i saw really skinny guppies with really fat ones!! i thought it was a disease?

yeah, i have one black male who's bigger than my biggest female & one female who's the same size as the smallest male... but the difference is mostly in length, not fatness.