You mean jag älskar katter. What you wrote means "I love the cats'", note the possessive.
Whats the difference between the "cat eats the fish" and "the cat is eating the fish" in swedish
Sure, that'd be katten äter fisk, or katten äter fiskar if you mean the plural fish.
Can this mean either "The cat eats the fish" or "The cat is eating the fish"?
Yes, Swedish doesn't have a continuous tense, so the same phrase corresponds to both.
fisk is an en-word and follows the normal pattern for en-word definites.
- fisken means the fish as in a single fish
- fiskarna means the fish as in more than one fish
I find it very irksome that 'the cat is eating fish' is incorrect despite it being perfectly correct in English (and also according to some of my Swedish speaking friends). 'The cat is eating the fish' makes it sound like it's eating a specific fish, and sounds completely unnatural
"the cat is eating fish" would always be katten äter fisk, though.