Translation:There are a lot of fish in the pond.
I think (correct me if I'm wrong!) because さかな is marked with the subject particle が, the sentence is really about the fish - there's a lot of them, and they're located in a pond.
If the pond were the subject, then there'd be a stronger case for assuming the listener knows which pond you're talking about, so you'd use the. But in this case, you just know the fish are in a pond. Could be 'a' pond (new information) or 'the' pond (one the listener knows about). I don't think there's enough context to assume one over the other, and without the は particle a pond might actually be preferred - I don't know the language well enough to be sure though!
"A lot" can be singular or plural, and where I live "lots" sounds like slang.
Fish is countable when referring to the animal, and uncountable when referring to food, as Alcedo-Atthis said above.
If we want to refer to living fish in a pond, we should say "there are a lot of fish".
I think the word "fish" makes it extra confusing because it's exactly the same when it's countable and uncountable, so your ear is used to hearing it both ways. If we were talking about chicken, I think it's clearer. "There is a lot of chicken" (food) vs. "there are a lot of chickens" (animals).
Short version: が places clear emphasis on the subject. は is a more general topic marker.
が is always used after question words, e.g. だれが来ましたか ("Who came?") as well as in the responses to such questions (松岡さんが来ました -> "mr/ms Matsuoka came"). If you write 松岡さんは来ました it's understood that Matsuoka was already expected to come, and this information could also have been left out (i.e. just 来ました would have sufficed).
There are also certain verbs that nearly always use が, such as いる・ある (to be), いる (to need), わかる (to understand/know) and できる (to be able to/to succeed). In these cases, the 'psychological' subject can be marked with は, while the grammatical subject takes が. E.g. （わたしは）ぺんがいります: "I need a pen". This is demonstrates why applying the grammatical rules of one language to a different one doesn't work; in English, "I" would be the subject and "pen" the object, but in Japanese the pen is the subject (it is being needed), while "I" is merely some extra info that might as well be left out.
Besides general phrases, は is used in sentences with (an implied) contrast between things, where the subject may not have an explicit emphasis, but can nevertheless not be left out from the sentence. E.g. フランスの子供はよくブドウ酒をのみます: "French children often drink wine" (in contrast to children from other countries).
For me "a lot of fish" means that I'm commenting on the large number of fish there are, whereas "plenty of fish" means I'm commenting on the fact that there's enough for whatever purpose we're going to be using the fish for. Thanks so much for answering, it's always interesting to hear how speakers of different flavors of English use the language.
Sorry, I guess my language wasn't precise enough, I agree that plenty would be 'more than enough', whereas "a lot of" doesn't say anything about whether there is enough or not, just that there is a a large number.
'Bay of Adequate', though... haha. Looks like a lovely place to live.