My daughter is 16, researched and got a 20 gallon tank this past Sept. She got and cleaned gravel, a few little statue things and fake trees, which were also cleaned. She filled the tank. Had the water tested, made adjustments with chemicals and ran the filter with no fish for two weeks. Its an aqua air filter. We bought a supposedly better heater too. The filter has a sponge thing, carbon sack and bio something on top...looks like white rocks...sorry for being non technical.
She bought 2 mollies and 5 tetras and placed them in the tank after the two week wait period and having the water checked. She has test strips and tests the water once a week. By my estimate she changes 2 gallons of water a week with tap water she tries to get the water about the same temp and the tank...78 degrees or so.
All was well but two weeks a go a tetra died. Two days ago she did her first cleanup. She removed 25% of the water, changed the filter sponge and carbon but resused the bio mass thing.
Tonight I find her crying. One by one her fish are dying. The heater is cycling on and off. I do not have a second thermometer to insure the one in there is reading right. The waters test fine. The fish seem to be gasping for air darting up top and sinking down. The Mollies are gone. We have one tetra moving around, one not looking well and a baby Mollie in a separate container but in the aquarium not looking good. Its awful and there is nothing we can do. What could be causing this? Its happening so fast. What can we do to find out what went wrong and insure it never happens again.
I see several things wrong here:
1) you used chemicals, never use chemicals while cycling a tank, whether with or without fish. Never use chemicals, period.
2) you waited 2 weeks before putting in fish, a tank takes 6-8 weeks to fully cycle, whether it is without or with fish.
3) I don't see that you used an ammonia source when you setup your tank, did you or did you just let it sit?
4) test strips don't get the level readings correct. I would get an API freshwater
liquid test kit, it is worth the price and it is more accurate.
5) changing 2 gallons of water isn't going to even touch the water parameters (ammonia, nitrite, and nitrate).
6) changing 25% water is better, but I would do no more than 50% water changes, this will bring down ammonia, nitrite, nitrate if those are up.
7) when you cleaned the gravel and decorations, how did you clean them?
8) did you add dechlorinator? Dechlorinator gets rid of chlorine in the tap water, chlorine is toxic to all fish.
I have a thread in the new to freshwater hobby subforum, called Correcting a cycle. I would advocate a fishless cycle if you have never had fish before. The link susankat gave is a good link to learn how to cycle a tank without fish.
There are hardy fish if you want to do a fish in cycle, some of them are danios, bettas, goldfish
. Bettas cannot be in a tank together, they will fight and kill each other.
I hope you will be able to correct your cycle.