Your tank is not cycled if your filter is only 3 weeks old and the toxins in the tank are killing your fish. Im assuming you are testing the water using a liquid water test kit not strips whcih are no good - if so buy one straight away. For goodness sake do not clean out or change your filter or you will make it worse - filters take 6-8 weeks to mature enough to break down all the toxins in your tank! You only clean your filter media (sponges) either once a month or sooner if they are gunky by rinsing them in the tank water you syphon off when you do a water change. if you run them under the tap in water that has not got dechlorinator in it (which I hope you have put in your tank when you filled it as that will also kill your fish)
Read this carefully - ive copied it for you from a thread on this forum
How to Save the Day (and the Fish) with an Un-Cycled Tank
OK, so you didn't do Fishless Cycling or you did scrub out your matured tank and now your fish are dying and nothing's breaking down that ammonia and nitrite. What do you do now? unsure.gif
Here's what I do to give my fish the best chance:
1. Immediately do a 10-15% water change with dechlorinated water and continue to do this at least once daily until your tank is cycling (i.e. ammonia and nitrite are at zero).
Change that to 50 - 75%, and it would be much better advice. Just make sure to match the temperature as best you can and dechlorinate the fresh water.
2. Test the water daily for ammonia and nitrite until the values are holding at zero for several days running. If levels are high, do an immediate, extra water change.
Good advice, but for the avoidance of doubt, 'high levels' is anything above 0.25ppm for ammonia or nitrite. You can effectively ignore nitrate for now.
3. If at all possible, get some matured filter media and/or gravel from a matured tank and put it in your tank, suspended in an old stocking. This will hopefully impregnate your new tank with the beneficial bacteria. One lady successfully used floss from a relative's fish pond to colonise her new tank! (you have to be careful that what you use is clean and free from pathogens, of course).
Again, good advice, but the media really needs to go in your filter, not just sitting in the tank.
4. Keep good aeration in the tank both to help the fish a little and to oxygenate those beneficial bacteria.
Good advice. For the avoidance of doubt, aeration occurs at the surface of the water, so anything which moves the water surface helps.
5. Avoid using medications, if at all possible, as many medications kill off beneficial bacteria. Your fish may well get ick, fungus or other infections due to the stress of the ammonia and nitrite but the priority is to get that water quality as good as possible.
This is absolute nonsense. If fish are ill, they need treated with the appropriate medication. Whilst bad water conditions can be the cause, and it is important to fix the water conditions, the illness itself also needs treated or the fish may die.
6. If you have delicate fish in the tank, such as plecos, corys or other bottom dwellers, tetras, pencilfish etc. try to re-home them temporarily, such as asking the Local Fish Shop to look after them until your tank is cycled (after all, chances are that they got you in this mess in the first place).
Even better, re-home all the fish and perform a fishless cycle. This eliminates the chance of the fish being poisoned to death in their own waste.
7. Live plants can directly use ammonia, so if you can, put some cheap aquatic plants in the tank, such as elodea or giant vallis.
8. Don't feed your fish at all if your ammonia readings are high, and only feed bare minimum rations every other day, until the tank cycles. This will cut down on the ammonia the fish produce. Since fish are cold blooded creatures and don't need the calories of a mammal they can go several days without food anyway, and the occasional fast is good for them. Your fish may not be very hungry anyway so do be careful not to feed more than the fish can eat and clean up uneaten food immediately, before it rots and produces even more ammonia.
Great advice!!
9. Only clean the gravel superficially, of obvious dirt and uneaten food. You want the bacteria to colonise it and actually start to grow. Also, don't swap out your filter at this point - if it gets blocked, just clean it enough to unblock it, in used tank water.
Regarding the gravel, this only applies if you use an under gravel filter. Otherwise, clean away. Regarding swapping the filter, good advice, swapping the filter would be the worst thing you could do.