Well your tank is uncycled. Nitrate is fine as there is probably none there. I would suggest reading the sticky on the newbie forum about cycling a tank. 3 days does nothing as you need an acceptable level of bacteria that can only come about with a food source, which is ammonia. While a nitrate test kit is good ammonia and nitrite are more important. If you tested for those one whould be high, most likely ammonia and that is killing your fish by burning their gills.
the first thing you need to do is a water change. As I don't know your water params I would suggest starting small at first but often. Start by doing a 20% water change every hour for the next few hours. That should help to redue the ammonia and any nitrite down to tolerable levels. Until you get more test kits I would suggest doing 50% water changes (using dechlorinator of course) to keep those levels low. If you do have test kits then keep the ammonia lower than 2ppm and the nitrite lower than 0.25ppm and your fish should be okay.
Next don't add any more fish until the ammonia and nitrite levels stay at zero 24 hours after the last water change and yor nitrates are climbing. That means that your tank has a sufficient level of bacteria to deal with the fish load you have. Any new fish will cause a spike in the cycle but test the water at that time and do water changes if needed.
Also feed your fish only every other day right now (at that just once per day, not several small feedings). The fish can survive without food for a day and that will reduce the waste in the tank, thus reducing the ammonia buildup.
I don't know your tank size but in the future stay away from plecos unless you have a really big tank. They should be able to grow up to 18 inches long and create so much waste (poop) that they are counter productive in small tanks. Plus an algae eater isn't needed. Weekly water changes of 30-50% (once the cycle is settled) and an algae scrubbing brush is all you need.
The redtailed shark also gets quite large and unless you have at least a 20 gallon tank and really a 30 gallon tank I would suggest seeing if the fish store will take it back. It should be fine for awhile but will have its growth stunted which will lead to an early death for it.
Good luck.
