Colloidal silver causes disease!

[Synirr]

None of those pictures were doctored. I've seen a documentary on it and argyria is quite real and can, infact, turn someone a very unnatural shade of blue, grey, or silver. This is medical fact.
That said, it's not very common and I would think you'd have to take extremely high doses for an extended period of time to experience such a side effect. Also, silver protein is more likely to cause argyria than true colloidal silver (but note that it is sometimes sold under the colloidal silver name).

From http/www.emedicine.com/derm/topic595.htm:

The most common cause of argyria is mechanical impregnation of the skin by small silver particles in workers involved in silver mining, silver refining, silverware and metal alloy manufacturing, metallic films on glass and china, electroplating solutions, and photographic processing

The normal human body contains approximately 1 mg of silver; the smallest amount of silver reported to produce generalized argyria in humans ranges from 4-5 g to 20-40 g. Silver at 50-500 mg/kg body weight is the lethal toxic dose in humans.

I don't think the change in colour is the result of silver replacing the iron in our blood, but rather, the result of the silver particles being deposited and stored in our skin. Given that, it may not even be possible for fish to get argyria, depending on whether or not they store silver in their skin.
As a side note, bettas deposit waste products of nitrogen metabolism in their skin, and this, depending on if the wastes are stored in a crystalline form in iridocytes or in granular packets, results in an iridescent or opaque betta