Okay..so since most of the things you see in your tank are insect related.. i've decided to add to this thread with my own rendition (on top of rain's) so as to help you guys out a bit more..Thanks to Jason for allowing me to use tonnes of his photos from http://www.troutnut.com/. It will go a long way in helping to explain and identify what you have in your aquariums. Take note that this is a rough guide so i won't delve into details..but it will give you a general idea. So let's start off with..
Beetles - Coleoptera
Since it is relatively easy to tell how a beetle looks like..i won't elaborate on water beetles. There is however, one type of beetle larvae that i would like to point out, something you probably will never associate to it growing up to become a beetle since it looks so bazaar. Sometimes i find it in my pond.
Commonly known as water pennies...beetles in the family Psephenidae have aquatic larvae that look...well.. like pennies in the water.. Here are some pictures of them..
Looks pretty cool doesn't it?? And yes.. it grows up to be a beetle..
Larvae cling to the underside of rocks in fast-flowing streams and graze on algae
Next we move into the ones most aquarist will worry about...
True Bugs - Hemiptera
Lets first start of with this big fella... i don't think many of us will get this in our tanks but i'll just put it up for easy identification, just in case, one of us does.
The bug below is also commonly known as the Giant water bug and belong to the family Belostomatidae. These bugs are big!! i have seen some that are about 5- 6cm or more in the wild. They do feed on fishes and sometimes even small frogs!! so they are predatory..also if you want to catch them out of your tank...please be careful.. they do "bite" and it can be awfully painful...basically they inject you with their mouth part and pump a whole cocktail of digestive juices into your finger... doesn't sound fun does it?
Next is something very familiar..
the waterboatmen, these belong to the family Corixidae. They feed on Algae and very minute aquatic organisms..so maybe fry will not be in danger... Here is a picture of one..
Okay.. dont be confused..These next few pictures may look like water boatmen but they aren't.. they are backswimmers, family Notonectidae.
These ARE predatory so they will eat your fry.. some will also "bite" and it can also be quite painful. The difference between these guys and waterboatmen?? well.. they aren't called backswimmers for nothing.. they swim on their backs!
Bottom view - they swim with this side facing up!!
Lastly, to end of section 1, we have the water scorpion , family Nepidae. These also are BIG..but will rarely be found in your aquarium. However, these pictures are just in case. As you can see.. they have those raptor-like appendages that they use to hunt..so yes.. they are predatory and yes.. they will also "bite". Easily identified by the siphon they have on rear of the abdomen. They use this for air.
whew!!.. so i'll stop here for now before heading into the more confusing stuff..haha..more updates tomorrow..