we rescued two fish from living in the dark in someones garage and they've been happily introduced to two of our own for over a year but recently we replaced the florescent light, which they seem to love and are now much more active but within days one of the rescues has started eating his rescue pals fins causing him to spend most of the day hiding in the weed. why has the fish turned? 

Fish, like many animals, can sense when one another are harmed. Sometimes they will go after that fish, nipping on it, or they will ignore it.

Are you able to tell if there is anything wrong with the fish? If there is, try to get that treated, as it is likely the reason he is being picked on.

Do you know if your fish are male or female? Sometimes males will nip on females, though not to the extent of eating a fin.

If the problem persists, I would suggest putting your fishy friend in either a separate tank, putting a divider in your current one, or keeping him in a floating basket. Hopefully this would help him get back to full strength, and maybe you could even find out what is wrong while he is in there.

Hope I can help,
Pearlscaled

Elwynnsolo is right, single tailed and fancies shouldn't be mixed.
The single tailed fish are much faster and will out compete for food and can cause the slower moving fancies stress from their quick movements.

I would just like to add that goldfish shouldn't be released into rivers or streams. They should be re-homed, to someone with a suitable tank or pond, or perhaps even a botanical or garden centre that can take them in.
It is illegal to release non-native fish into local waters. If you think you live in an area that has native waters, then please check with a governing body or environment agency first!

Although I can appreciate Elwynnsolo if you live in an area that has native waters for goldfish and it was safe for you to release Groucho, I just wanted to put the information there just in case

Do let us know how the fish are doing Barroniom