hello laurieangel.
welcome to koko's!
would you happen to be able to provide details about your tank?
[*]Test Results for the Following:
Ammonia Level?
Nitrite Level?
Nitrate level?
Ph Level, (If possible,KH and GH and chloramines)?
Ph Level (KH/GH) out of the Tap?
Brand of test-kit used? (strips or drops?)
[*]Tank size (How many Gals) and How long has it been running?
[*]What is the name and size of the filter/s?
[*]How often do you change the water and how much?
[*]How many fish in the tank and their size?
[*]What kind of water additives or conditioners?
[*]Any medications added to the tank?
[*]Add any new fish to the tank?
[*]What do you feed your fish?
[*]Any unusual findings on the fish such as
"grains of salt",
bloody streaks, frayed fins or fungus?
[*]Any unusual behavior like staying at the bottom, not eating,ect..?
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also when was your last water change?
2 goldfish and an african frog in a 10 gal is a bit much... more than twice what the tank is suitable for, so it is imperative that you perform water changes
at least once a day. depending on how long your tank is set up and the water parameters... the amount of water being taken out for each water change varies.
some helpful pages here on koko's that would definitely help you out are:
http://www.kokosgoldfish.com/tensteps.htmlhttp://www.kokosgoldfish.com/care.htmlfrom the sounds of the "black" on the fringes of the fins/tail it sounds like ammonia burn healing which can be quickly solved with good water quality. many of the things that can afflict goldfish can be solved by keeping their water in good condition and performing water changes as often as needed.
a lot of this information may seem a bit overwhelming at first, but we're definitely glad you're here seeking help for your goldfish!

of course it's not absolutely certain that your goldfish has ammonia burns, so in order to get a good diagnosis of what maybe causing this behavior and color change, please try to answer the questions above!