I can not speak of tinfoil barbs, as I have never kept them (I have not had tanks large enough...)
Regarding Silver Dollars I completely agree with the idea that the longer the tank the better. However, there are important modifiers to that concept -
- Type of Silver Dollars (speaking about things like wide bars SDs versus say Metynnis species is completely different!, and almost can't be mentioned in the same sentence! - and this is just an example). Bottom line, SD's are not all the same.
- Aquarium conditions - No plants and too much light --> almost certainly skittish silver dollars scurrying to hide and darting about (regardless of aquarium size)
- Number of SD's and sex ratio within the group - a critical element often not considered. Too few --> skittish. Too many or unfavorable gender ratio --> skittish or too much fighting leading to undue stress.
In the picture, a relatively harmonious group of 8 Metynnis (3 species) in a standard 75gal tank. After 3 years, I ended up separating 3 males out into another planted 75g, because of too much sexual tension (not because of skittishness!). The group will soon reunite (hopefully with a couple of additional females) in a 6-foot planted tank, currently being setup.
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