Recently, my mosquito rasbora (Boraras merah) was acting weirdly by hiding in a corner and sleeping more than usual.
Upon closer examination, I found “specks” on it. At first I thought it was velvet. However, this was an old tank and there has not been any introduced fish for more than 2 months. Then I realized it was probably dropsy and the specks were actually pineconing of the scales, which looks like specks of dust because of the smallness of the scales (this mosquito rasbora is one of the smallest fish in the freshwater aquarium, measuring less than 2 cm when fully grown). In conclusion, I think dropsy in tiny fish can probably look like velvet.
I think the reason is that the dwarf hairgrass (and Japanese hairgrass) planted in the sand substrate has failed to grow, and in fact has started to die and rot, releasing ammonia, nitrites and/or nitrates into the water. I do have a Seachem Ammonia Alert badge that did not show any readings.
Hence, I suspect it is probably nitrates. I added Methylene blue (full dose) after 50% water change, and then continued 50% water change on the second day, topping up the Methylene blue (half dose). On the third day, another 50% water change. (For all days during the water change, I removed the rotting hairgrass as well as any pond snails pests that I find, and wiped off algae).
After the third day, the mosquito rasbora began to behave normally again, swimming around and schooling together with the lambchop rasbora. I hope it will survive.
The lambchop rasbora seem unaffected and seem more hardy than the mosquito rasbora. I think the mosquito rasbora may be more sensitive to water parameters.