What is That?

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As I was using the rapid-shoot on my digital camera to try and get some better photo's of my guppies (I didn't exactly succeed in getting any non-blurry images BTW) I did came across an interesting find which I hadn't noticed until I was later viewing over the images on my computer. As you can see from the above image here there is some type of creature which dwells in my aquarium... I am not 100% certain as to what this is (perhaps planaria?). Thus far I haven't been able to see these creatures with my own eyes although I have been able to spot planaria before. Interestingly enough certain cameras are capable of capturing things of which we can't see with the naked eye, and the guppy is capable of seeing them.. and even eats them as you can see from the slide of photo's I took below (you may have to look closely!).