Hi and welcome to the forum
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Never heard of a buttercup fish before, but i suspect it is one of the many inventive names that petshops give to certain varieties of goldfish. Does the fish in question look like any of the goldfish types pictured in the link below;
http/www.bristol-aquarists.org.uk/goldfish/goldfish.htm
Generally speaking, you need to have 20gallons for the first goldfish and 10 for every one after that when it comes to fancy goldfish (although a lot depends on the exact type of fancy goldfish in question)- for non-fancy goldfish like comets, commons and shubunkins, you need at least 75gals for the first goldfish.
Danios are technically sub-tropical fish (temps are 19-23degree's are good for them), although they are hardly and adaptable fish and a lot of people keep them in tropical tanks (24 degrees or above). They are shoaling fish and need to be in a group of at least 6 of their own kind to thrive best, and in a tank of at least 12gallons long since they are very active fish; in tanks or groups too small, these fish tend to not live very long, or show abnormal behavior, or simply get ill a lot etc- so giving these fish the right type of set up and fish stocking is very important for them for them to thrive and live long, health and happy lives.
Goldfish will eat any fish that can fit into their mouths, which basically means that most small fish are under threat from goldfish considering that most fancy goldfish can grow to 5-8inches long and non-fancy goldfish to 12inches+.
Fancy goldfish though are quite slow due to the odd body and fin shapes they have often been bred to have. So some people keep small active and fast fish like WCMM's (white cloud mountain minnows) or danio's with them because these sorts of little fast fish are often too agile and quick for the fancy goldfish to catch and eat. Some people are able to keep fish like these with fancy goldfish for years and years with no problems whatsoever like this, however, it is not a completely risk-free type stocking to do and over the time i have been on this forum i has read numerous threads from people who have been shocked to find that their fancy goldfish have managed to eat various small fish like danios or WCMM's that they have put with the goldfish.
So the choice is up to you- personally i would advise against such sorts of fish stocking with fancy goldfish as it is not risk free. Non-fancy (or slim type) goldfish are definately too quick and agile for small fish like danios and WCMM's to escape from them and would definately be eaten by the non-fancy goldfish sooner or later.
If you are new to fishkeeping and are not familiar with "Cycling" tanks or the Nitrogen Cycle, i strongly recommend that you read these articles to understand more on the water quality in your tank, as keeping good water quality is one of the most important keys to keeping healthy and happy fish;
http/www.fishforums.net/index.php?showtopic=10099
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Do you have dechlorinator/water conditioner, and a filter in the tank at the moment?