Small creatures in my tank

Question
There have been these small creatures in my 40 gal tank that don't seem to
go
away.  I noticed them 2 months ago and cleaned out my tank.  I thought I did
a good enough job but they are still there.  3 days ago I put some medicated
tabled in my tank thinking it would kill them off but instead they seemed to
swarm.  I dropped in a single tablet in where they were swarming they
attached themselves to the tablet but didn't die.Im now worried that it may be
effecting my fish. 2 of my fish have did in the last week.One was a neon tetra
and the other a guppy.  The tetras have been in there since I got the tank
along with a algae eater and a mickey mouse fish.  All of them are still alive.  
The little creatures look like little balls with legs that for the most part stay in
and on the rocks and just move around and leave the fish alone till they die.  
The only place I think they could of come from is from the 2 live plants that I
have.  Help me please.  I don't want all my fish to die off.

Answer
Hi Krystal;

The bugs are actually just a harmless little water critter. Little fellas like those often exist in our tanks in very small numbers, it's natural. It's just that we rarely see them. When they start to overpopulate is when we notice. They aren't the cause of your fish deaths. Their overpopulation is a symptom of a bigger problem though. (I'll explain what may have happened to the fish in a little while.) These little critters thrive on decaying organic matter and consume excess waste in your tank. That's why they only attach to the fish after the fish die. They aren't interested in your fish while alive but are doing what they are designed to do; be a "cleanup crew". They can't be killed with chemicals or medications and it isn't necessary to try to kill them anyway. They have to be starved out by removing their source of food. The excess waste needs to be removed and cleaning the whole tank probably took care of that. Just avoid overfeeding and don't overcrowd the tank, as they are causes of excess waste.  

As strange as it may seem, the probable actual cause of your fish dying was from cleaning out the whole tank. It disrupted important colonies of beneficial bacteria and caused your tank to go through a "break-in period". It is a very stressful time so your fish became sick and sometimes fish will even die from it. The break in period can take up to 8 weeks and fish don't always die right away. The toxins that rise during break-in can cause internal problems that you can't see but the fish still dies a little while later.  

All tanks need a weekly 25% water change along with gravel vacuuming to avoid these kinds of problems. Do weekly partial changes instead of a stressful major cleaning and your tank will remain more stable and will provide a healthier environment for your fish.

Good luck and I hope the rest of your fish are okay...

At Your Service;
Chris Robbins