What you are describing are not aphids, they are aquatic
insects called springtails of the order Collembola. They are
completely harmless and were probably introduced with some of your
floating aquatic plants (possibly collected from the wild). They
will colonize the floating plants in your tank and probably go
unnoticed unless the plants are disturbed and they start jumping
around. The fish will eat them if they can catch up with them, but
be careful that an enthusiastic eater doesn't jump out of your tank.
Jeff Scott, Ph.D.
Vista Technologies, Inc.
jscott at hsv_vista-inc.com