Fish dieing within a day.

Question
First I just listed all the stuff that you would want to know about my tank.
-Been set up for about 2 months
-10gallon
-random fish of all sorts (read below)
-aquaclear filter
-pH unknown, ammonia and nitrite are fine
-I don't change it too often becuase all my fish keep dieing before it gets dirty.
-freshwater

Hello I have a small 10 gallon tank that i generally use for experimenting with my new fish.  But I haven't had the greatest luck with it.  I pulled it out of my basement a couple months ago and it has a mirror on the bottom of the tank.  Over the past months I have been using it, it has killed all my fish within a day or sometimes two.  Such fish (and others) include: a betta, several feeder guppies, crayfish, snails and even local waterbugs.  I also have local plants in there like duckweed and lake weed.  Anyways, the only things that have seemed to survive these random occurances are the plants and snails.  My only guess is that it could be the pH or some disease.  I have tested for the ammonia and nitrate levels so it couldn't be that.  I was also thinking that the glue used for the glass may be toxic... I don't know I just pulled it out of my basement.  A pet rat used to live in the aquarium before we put it away but I cleaned it out before i used it.  Oh, one thing that I also noticed is that there is a shiny substance (kinda looks like oil) on top of the water.  It sat for a couple weeks without filtration or anything and the substance began to clump together if I troubled it.  There is a lot less of it now that I have filtered and removed some water.  How it got in I have no idea.  Anyways thanks for reading this and I hope you can narrow it down to something soon.

Answer
Hi Mike;

It might be a good idea to check the pH. Also test the pH of the water your new fish are in. If it's more than a point different, such as yours maybe being 8.0 and the store water is 6.8, it can cause pH shock. Fish can tolerate a slow change in pH over several hours but it they are thrust into water that is too far from what they are already used to, death can be the result.

Also be sure to slowly acclimate your new fish and other water pets. Float their transport bags in the tank for 10 or 15 minutes and add a little tank water to the bags 3 or 4 times during the acclimation time. Net the fish from the bag  and gently release them to the tank. Throw away the transport water and don't add it to the tank.

Your new tank has just been through the break-in period too so it's possible that the ammonia and nitrite levels that  will rise during that time simply went down before you could  detect them on a test kit. Try adding just one small fish and see how it does. Don't add any more than one and test the water for ammonia and nitrite every day for a week. If that works, add another fish and wait another week. Keep doing that until your tank is at the population you want. Be sure to make weekly changes of 25%, or more often if ammonia and/or nitrite goes too high.

I'm thinking that if there was something toxic about the tank that killed the waterbugs, fish and crayfish, the snails would die too. Unless it's something that doesn't affect them. Unlikely, but not impossible. Sometimes a toxin can be absorbed by the aquarium sealant even if the sealant itself is safe. If the above suggestions don't work you might consider stripping the sealant and resealing. Be sure to buy 100% silicone sealant that is non-toxic when cured and safe for aquariums. Fish stores and hardware stores usually have it.

Good luck.....

At Your Service;
Chris Robbins