04-06-2014, 09:45 PM
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It's possible you could have an internal parasite. Especially if they're eating well, but are still getting skinny and are hungry, and then dying off. Try feeding them twice a day and see if that helps. I've found feeding them once a day, they're usually absolutely ravenous by feeding time, and not all get what they need because everyone is starving.
As for knowing age: This is difficult. You never know how old fish are at shops, usually they have juveniles, fish that are 5-8 months old, but sometimes they do give away much older fish. Providers often give away their breeders once they've had them around for a couple years.
Sometimes they will be a bit slower and look a little gaunt, but they don't just drop weight like you're talking about, not usually.
I recently had a swordtail who passed of old age, he was a bit slower, but otherwise got around ok and ate well, who passed away. Just went to sleep in his little place between my plants, looked normal until I realized he wasn't moving to come for breakfast. Not all get this way though, some never really show much age, but just die like that for no apparent reason.