My wife has had an aquarium for about a year and a half. Nothing fancy 20 gal. tank with a couple of Swordtails, 3 or 4 Danios, and two snails. The other night she decided to clean the tank and do a full water change. She had been wanting to change the look of the tank (we have had some pretty natural elements in there for a long time). She decided to add a lot more brightly colored decorations. She changed the old neutral substrate out for some new brightly colored substrate; she also added new brightly colored plants. Basically everything old was taken out and replaced. We rinsed and washed everything as we did when we first got the tank. We did a full water change using distilled water (we've done this several times and never had a problem). We added some Tetra Cleaning Bacteria to the water. We then added the fish and all seemed normal. The next morning all of the fish were huddled in the corner closest to the heater. Since they were close to the surface I believed that maybe they were lacking O2 (even though we have three bubblers in the tank). So I removed a little bit of the water to allow the filter to agitate the water a little more. By the afternoon two fish were dead. This morning they were all dead. I told my wife that we may have changed too much too fast....and that the substrate may have hidden some goodies that the fish needed. I'm looking for any advice before we buy new fish.