Chances are the deaths are to do with ammonia poisioning and your tank has now cycled but the fish have sadly sucummed over time. However chinese algae eaters/sucking loach are a "clean up" fish, and will feed on dead and dying fish not just algae. Usually around 3-4" they start getting the taste for other foods than algae and in an aquarium its sadly usually fish slime coat. Deep bodied fish like the angels and gourami would be a target the bigger it gets. These are one of the sorts of fish I am asked to re-home most often. The chances are all your fish have some degree of ammonia poisioning due to the fish in cycle, which clean water is the only answer for, but with weakened immune systems they will be a target for the algae eater, I would suggest taking that one back to the shop I am afraid.