Usually a fish will not eat or swallow something that is harmful. They will take it in their mouth and spit it out to test it. I doubt the spider was harmful and the white belly is unrelated to eating the spider.
King Dedede has gotten weird colors on his anal fin for a while and it's now spreading to his tail! the colors on his anal fin were: orange, purple and pink. the purple has spread to his tail today. HELP!!!