Ры́ба can be used as a mass noun. But your variant is also correct.
Of course his variant is correct too, that's exactly what I've said:
But your variant is also correct.
«В пруду плавает рыба» can either mean there's a single fish (рыба used as a countable noun), or that there is an indefinite number of fish (рыба used as a mass noun).
https://en.wiktionary.org/wiki/%D0%BF%D1%80%D1%83%D0%B4#Declension I was curious myself. Wiktionary has it listed as locative. I thought there were only six cases in Russian with some irregular declensions that are remnants of older cases. I once read researchers stated there were up to ten cases in Russian. I can't say for sure.
I don't know if u were replying to me or JamesChat but I wrote: "Wiktionary has it listed as locative." For more related info on this, if u like: https://www.alphadictionary.com/rusgrammar/caseexcpt.html
Now this author, who is a linguist, lists it as Prepositional II. He does not give Пруд, but gives others: на полу
That's is very abstract and extremely generalized, in a way which makes your translation perhaps too cerebral - too removed from reality. I don't know whether Duo has accepted by now (two years after your comment), but to make your sentence more specific and more real, it should be translated as "the pond", not "a pond".
В данном случае fish — это множественное число.
У слов fish, sheep, fruit множественное число совпадает с единственным (впрочем, можно сказать fishes и fruits в значении ‘разные виды рыб/плодов’).
A fish (singular) swims/is swimming
Fish (plural) swim/are swimming
After some more research, I'm very uncertain about this particular word. While the singular рыба translates as "a fish", there is also the Russian word рыбы, which seems to translate as the plural English "fish".
So, it seems like it would be:
Singular: рыба плавает - A fish swims/is swimming
Plural: Рыбы плавают - Fish swim/are swimming
The singular subject and verb seems like it should only be "a fish swims/is swimming", since there is a plural version for both.
Unless, of course, Russian treats рыба like картошка, as a mass noun, in which the Russian singular subject and verb would be translated as plural English subject and verb.