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How do I tell if they're inbred? Had them for about 6 years and the only addition was 3 males about 6 months ago.
Well, after 6 years they're definitely inbred. Mutations are the main thing to look for. Inbreeding is why we have so many different varieties of these types of fish.
What should I look for?
Weird growths, really crooked spines, 2 tails, things like that. Fish are generally pretty resistant to bad mutations due to having such large genomes tho.
Only thing I've seen is a crooked spine, But only once
That is indeed a sign. It's difficult with guppies because they're such prolific breeders and reach sexual maturity at such a young age it's hard to separate them. At least not without running dozens of small tanks.
Soon I plan to get a 75 gallon with guppies should I purchase new ones or keep the current ones?