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).
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".
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
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.
(Recovered replies) 1: Рыба may be singular => the verb is "плавает"; - Рыбы may be plural (we can count their number) => the verb is "плавают"; - Рыба may be a mass noun (we can't count) => the verb is "плавает" again.
What's interesting is рыбы in the biology classification (рыбы, птицы, земноводные) use as plural - рыбы even though we can't coun them in this case.
2 ens5: Maybe not so illogical that the biology classification uses a plural form, since it is a table in which we are characterizing a large number of different genera and species? We may not be counting the number of fish in each species in this case, but we are implicitly counting the different kinds of fish.
I'm curious. If рыба is a mass noun representing a number of fish, could/should the verb be plural, i.e. плавают?? In English I think true mass nouns are treated as singular ("hair is..."), while the English word fish is really an alternate plural of the singular fish ("fish are...," meaning, of course, "fishes are..."). Could the use of плавает, here, imply that this is just one fish, or does this act just like an English mass noun, justifying DL's translation?