I recently joined this forum a few weeks ago when my own 20 gal started cycling and i started losing fish. I did 50-70% water changes daily for five days using stress coat full 20 gal dosage and jump start (suposedly a BB culture) full 20 gal dosage. I added an extra hang on back (HOB) filter for extra media surface area. My tank temp kept around 82 and pH was always higher than 7.4. Large air pump running through 2 small stones and one 6 inch stone for oxygenation. My tank cycled in 6 days and has had 0 amonia and 0 nitrites for about a week now. I also have several live plants so nitrites are anoit 20ppm. Knock on wood no losses in the last week and a half, not even a platty fry.
Sorry it got long winded, try using jump start and good luck! Don't add any new fish until your levels are stable for a week or so.