Fish Tank Problem

Question
I have a 29 gallon tank that I had a lot of problems with in early November where something killed off most of my fish in the tank. The water quality was always fine and I did frequent water changes through out. I finally brought a temporary 6.6 gallon tank as a last resort to save the remaining 4 fish, two little balas, a neon rosy barb, and a pleco. I went home for college break and left the 29 gallon tank empty. I wipped it down with tank wipes and through away all the scenery and gravel. When got back to school I refilled the tank and besides a little left over dead algae, the tank looked brand new. However, once I added the balas and the rosy barb, they started showing similar symptoms to how the others had died in the past (rapid breathing and shimmying in a covered location) They also seemed to be photo-sensitive, meaning they look and act more healthey when the light is off. Any ideas of what could have be dormant in my tank all that time. I moved the 2 balas back into the temporary tank and they are doing fine. I am monitoring the rosy barb closely. Also, during the last episode in November, the balas got a severe case of fin rot and I nursed them back to health with a combination of Maracyn and Maracyn 2. I'm not sure if that helped more or getting them out of that tank. Any thoughts would be greatly appreciated, even if you think I may just be paranoid after what happened in the past. Thanks

Answer
Hello Matt,  First I need more info. Do you have a filter on the tank? heater? Gravel or sand bottom? Important>>>>Prior to november when was the tank first set-up and then fish added???
  Get back to me.
  TROPICAL FISH KEEPING 101....But I'll assume the tank was new in October. A newly started tank will have problems. Unavoidable. The list of what can happen is too long. Fish eat and excrete waste. Much of the waste along with uneaten food remains in the gravel. (A.K.A.garbage). As the tank ages good bacteria grow and feed on the garbage and eliminate most of it. The filter cleans the water of some more junk. It takes time for those helpful garbage eaters to get established. Usually at LEAST 4-5 weeks. To make things worse, evil bacteria and other nasty critters grow in the waste first. Just waiting for a fish to infect. The garbage then produces AMMONIA (NH3). Ammonia KILLS FISH. NH3 is the number one reason for fish deaths in an aquarium. And most of the NH3 is a direct result of you overfeeding the fish. Every new aquarist makes that same error. Fish are not people and do NOT need 3 meals per day. They will come zooming over when you go near to feed them simply because it is easier for them to grab the new food than search around the tank for more. Lazy fish let 'em search.
   Ok lets start over....You need>>> tank, proper size filter, heater, top, light, PH test kit, NH3 test kit, gravel in whatever nifty color you like....oh geeeezz not neon purple. Oh and water.
   Put the tank in the tub/shower and fill it with warm (not hot) water. Add a cup of salt and wash the inside then bucket out the water. NOTE:...NEVER USE AMMONIA OR CLEANERS CONTAINING IT TO CLEAN THE TANK. Ammonia will degrade the silicon seals. windex and other glass cleaners contain NH3. Use water only on the outside.... Rinse it a few times to get the salt out. If some salt stays thats fine. Set the tank back up in the desired location and get everything running, filter, heater, light, etc. Do NOT add fish. Let the tank run for a week. After a week add 2 or 3 fish. Feed them once per day only and much less than you think they need. These 2 or 3 will get the bio-system started and helpful bacteria growing. Stick with those 3 fish for the first 4-5 weeks. After that buy a couple per week. Adding 15 fish all at once results in a quick flood of NH3.
Fin rot is indirectly caused by stress from various things. The maracyn may have been an over reaction and too late. Leave the remaining fish in the small tank until the 29 gets 3-4 weeks running. Moving them again too soon will only make them more stressed.
   Add some live plants to the tank. They help the water quality a lot. Some fish will feed on and destroy the plants faster than the plants can grow. Buy some JAVA FERNS and AMAZON SWORD. Fish will not eat those and they are easy to grow. Plants require 8-10 hours of light per day.
  Put the decor back in.
  While the 29 is getting adjusted make a 4 gallon water change twice per week for 5 weeks. The water will probably get cloudy but don't worry, it will go away in a week. After 5 weeks cut the water change to 5-8 gallons once per month.
  While you are doing all that...LOL.... Test the water weekly for PH (acidity) and NH3. The NH3 (best is 000)wont go up for a while. The ideal PH is about 6.8. If it goes up don't worry. It will go down in time. If the PH gets below 6.0 add 1/2 teaspoon of Baking Soda per week until it gets to 6.8. You can buy stuff to raise the PH but all thats in it is baking soda, water and some pretty blue dye.
Your problem happened a second time because you repeated the mistake....added fish to the "recleaned and made new" tank too soon.
u can email me direct for help at....
  hancallaway@yahoo.com