Re: Aphids

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