aquarium problems?

It seems that since I did my partial water change last weekend problems in my aquarium started to surface. Water parameters are all ok including temperature. Just last night one of my male guppy died.
- Since last weekend my guppies doesnt swim to the bottom and only stays at the surface on one side of the aquarium.
- This morning when I turn the light on, I saw some small insect looking with white color jumping in the aquarium from stones to the glass. It's a cotton like but very small. Also, I saw one insect shrimp looking swimming near the bottom. Is my water contaminated by any bacteria or parasite?
- One of female guppies have a teared fin and have long thing attached to her anal. Also her belly is in reddish color.

After the water change I didn't put anything new, all the plants and decor are the same. The only solutions I put is the water conditioner, bacteria supplement (I've been using this almost a month ago), and algae destroyer (I just recently use this, first time)
Answers:
Sounds like you have a few things going on here that are not good.

First - you say your parameters are all good (I assume this to mean that your Nitrogen Cycle is fully completed - Ammonia reads 0, Nitrites read 0, etc.). Why are you putting a bacteria suppliment into your tank if in fact the cycle is complete? Do you not have any kind of mechanical/biological filter on this tank? Also - how big is the tank, and how many Guppies/other fish are in it?

The insect looking white thing and the one swimming at the bottom of your tank do not sound like the same thing. My fear is that you have Ich in your tank's gravel bottom (and the free swimmer you saw is a rarely sited instances of it - it will seek out a host (a fish) to attach itself to, and then goes into a 3 stage process before it's back to that free swimming stage you saw it in).

As for the insect and the red belly - it sounds like you have some sort of bug (Gnat) that had gotten into the tank's lid, and the red belly could be irratation from either water parameters not being fully cycled, or from her rubbing it on gravel/plants/sides of the tank which is an indicator of a parasite (most likely).

First - if your tank is not fully cycled - for future reference if you end up starting over again - I recommend doing a fishless cycle. Here is an absolutely perfect article on how that is achieved.

Fishless Cycling.
http://www.tropicalfishcentre.co.uk/fish.

The other article I would recommend you read through completely is probably the best publication on the Internet regarding "Ich" and how to treat it (and how not to treat it). I urge you to read this through when you have time. It will save you more time and money in the long run.

Ich.
http://aquafacts.net/wiki/index.php/ich.

Best of luck to you

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You've definitely got a parasite problem. You need to do a full water change, clean all the surfaces, plants and decorations, and then once you refill the tank, treat the water and fish with antibiotics. There are very detailed in instructions on the antibiotic label, follow them completely or else all the fish in the tank will perish as well. Act fast, or the fish will only get worse!