A few weeks ago I bought two guppies and one died. I've been feeling bad for the remaining on his own so today I bought two more. However, the origional guppy won't stop harassing them! He chases them around and darts at them, I can't tell if he's biting or not. I've never seen guppies do this to each other before. I had a couple that would gang up on other fish and eat their slime coat, but this is new to me. Is it a big problem? Think I should sepeate them?

I would say no, dont separate them for the moment, though I've never had guppies. I would only separate if they start injuring each other. One of my danios was pretty aggressive when I first got them, but now things have smoothed out a bit.

Perhaps you should remove the 1st guppy, rearrange the tank, do a large water change, and then home them together. Perhaps your guppy is feeling territorial and a change in water & decoration will make him less aggressive.

Today things seem to have calmed down and all 3 guppies are looking healthy. The older one still seems 'cranky' but not nearly as bad as he was yesterday. The waterchange and rearranging is a very good idea though, I may try that, thanks!