Depending on how you count it, there are three major patterns for verbs: those in unstressed -ω, those in stressed -άω/-ώ, and those in stressed -ώ.
κολυμπάω/κολυμπώ belongs to the second of those groups.
stress on stem:
- λύνουν (λύνουνε)
stress on ending, -a-:
- αγαπάω, αγαπώ
- αγαπάει, αγαπά
- αγαπάμε, αγαπούμε
- αγαπάνε (αγαπάν), αγαπούν (αγαπούνε)
stress on ending, -e-:
- θεωρούν (θεωρούνε)
Forms in parentheses are less common and may be vernacular or poetic; forms after commas are alternatives where both are reasonably common.
The first and third groups are pretty much the same, except for the position of the stress and the second person plural εσείς forms; the second one is a bit different because you see an -a- in some forms.
These come from verbs whose stems end in -a or -e before the endings, where those vowels fused with the ending in many cases, so an original θεωρέω turned into θεωρώ and αγαπάεις turned into αγαπάς, etc. (There used to be verbs in -o which contracted as well, but in Modern Greek those pretty much all got regularised into -ώνω, which inflect after the first pattern. So you have e.g. δηλώνει and not δηλοί.)
This also explains why the εσείς form is different in the first and third groups: a proto-form similar to θεωρέετε (with -ετε ending) contracted the two vowels to a "long" vowel: θεωρείτε.
The conjugation of είμαι is irregular but is similar to passive conjugations, e.g. λύνομαι λύνεσαι λύνεται λυνόμαστε λύνεστε (λυνόσαστε) λύνονται - αγαπιέμαι αγαπιέσαι αγαπιέται αγαπιόμαστε αγαπιέστε (αγαπιόσαστε) αγαπιούνται - θεωρούμαι θεωρείσαι θεωρείται θεωρούμαστε θεωρείστε θεωρούνται.
κολυμπεί is not an option, though, as the verb stem has an alpha at the end.
That's a bit like saying "it should be he flys as just adding an -s is more common and is more in line with how other verbs are conjugated"... but verbs in stressed -y turn it into -ie- before adding -s.
κολυμπάει should be accepted in an English-to-Greek sentence, though. (If not, report it.)
I'm not quite sure of the usage of the two variants. I think that αγαπώ, κολυμπά etc. are a bit more formal compared to αγαπάω, κολυμπάει etc., but both are used.
If the sentence's meaning was general, the sentence in Greek would be in plural, because that's how it's commonly used for countable nouns (τα ψάρια κολυμπούν, οι γάτες νιαουρίζουν, τα πουλιά πετάνε). Now that there is an article in singular, it is safe to assume that the meaning of the sentence is not general.
I looked 'κολυμπά' up on Google Translate (yes, I know it's not reliable) because the audio here is so bad. I wanted to hear another version. And I found that Google translates 'κολυμπά' as 'swim' or 'to swim' and 'κολυμπa' as 'swimming'. So I am wondering, since Duo has not yet covered gerunds when this is presented, why Duo uses the 'κολυμπά' form and translates it as 'swimming'?
κολυμπά and κολυμπά(ει) are interchangeable. The verb is used as a present or present continuous for "swim" so the translation for
"το ψάρι κολυμπά" and "το ψάρι κολυμπάει" can be
"the fish swims" just as well as "the fish is swimming".
It's just one of those language oddities. Hope this helped!