Corydoras Dying Off (SOS!)
I somewhat recently started my first freshwater tank, and I did my research and due diligence, at least as far as I know. The tank was cycled, and we started slowly adding fish to allow the system to balance itself.
We started with:
Initial load - 4 Albino Corys - 1 died pretty early after purchase and another died this past weekend.
Two weeks later - 1 Male Betta - from what we observe, he's fairly timid and non-aggressive
One week later - 5 Emerald Green Corys - 1 was found dead this past weekend and another looks unwell.
As of Saturday, these are the stats:
20g Long Tank
Plenty of hiding spots
Light on a 12 hour timer
Ammonia: Near 0ppm
Nitrite: Near 0ppm
Nitrate: Around 20ppm
Temperature: Around 78F until Saturday, Lowered slightly based on information found on the internet
Feeding: 1 Shrimp pellet per cory, twice daily. 3 Floating betta food things, twice daily.
Water Changes: 4 gallons, bi-weekly (Using water treatment to remove chlorine, etc)
I'd really rather not lose all of my corys, but I'm not sure what I can change to make them stop keeling over. I don't think the betta is a factor in their issues. I mostly see him run away from them, but he did nip a small amount upon first being added to the tank.
Any help anyone has to offer would be greatly appreciated. I've done everything I can think to do from a beginner's standpoint.