We have a 29g tank. It was cycled and going just fine since September 2012. About 2 months ago we started loosing fish. At this point we have 4 male fish remaining and have been stable for 2 weeks with no fish loss. For some reason we lost all the females but the males seem to be doing well. I cannot seem to get the tank stabilized. We do 25% pwc each week and gravel clean. We do have some algae that we clean as well each week. Ammonia is somewhere between .25 and .5. Nitrite is very close to 0. A very slight purple tint. We have high ph and it is 8.2. Nitrates are at 20. No plants. I do not understand why we cannot get the ammonia back to 0 with only 4 fish in a 29g tank. Water temp is 78-80. We have an aquaclear 50 hob filter with 2 stages of bio media, no charcoal. We have also swished the sponge in tank water to remove large debris. Can we do a 75% water change at this point to try and get the ammonia down? Any other suggestions? We do very small feedings for about a minute at most. Very little food makes it to the bottom. Should we add a catfish back in for bottom clean up? Any suggestions are appreciated.