Urgent - tropical fish help needed

[kickers22]

Just noticed that my fish in my well cycled tank are all coming up to the surface for air and gasping. DH cleaned out tank earlier and I suspect that he washed out sponges under the tap (ahhh) rather than in the tank water.
I have done a chemical water check and tests for nitrate, ammonia and nitrites are negligible but I wonder if he has wiped out all the good bacteria. Any ideas as to what I can do now, please?
In the tank which is 20 litres, I have 4 tetras, 3 shrimp and a Molly.

Is the pump on? Has he removed the chlorine?

Pump is on and he added the de chlorinator to the water he put back in after removing 20% or so. The only thing he did differently to me was wash out the filter sponges which he said he did under the tap. Would a second water change help or compound the problem, readysteady?

20 litres or 200 litres? If it's 20 litres then it's too small for the fish you have anyway. I'd keep doing the water changes for now- if the good bacteria is wiped out then you'll just have to wait for it to grow back. Keep checking the water every other day and start looking for a bigger tank; 60 litres would be about the right size for what you have.

If he didn't dechlorinate the new water then the chlorine might have burned the fishes' gills a bit. I'd do a large water change with dechlorinated water just to be sure. As long as the new water is the same temperature, pH and hardness as the water in the tank then it's OK to do massive water changes.

Sorry, x-posted. Don't worry about chlorine then. If he washed them under the tap then most of the bacteria will have been wiped out. Loads of water changes until the bacteria grows back will help your fish. It'll take a few weeks probably, depending on how much was killed off.

Thanks Readysteady - will do just that and look into a bigger tank.

Any friends or neighbours who have tanks and can offer you a squeeze of their dirty filter?! It's an odd request but it really does help!

Sorry - Eaurouge! Thanks for the advice.

I am phoning around to see if anyone locally knows someone with a tank. As they are coming up to the surface for air, would putting an air stone in help?

It can help a bit, but the best way to increase the oxygen is to make sure the filter is breaking the surface of the water.

Good luck,

Thanks everyone - fishes all survived and are happily swimming at their own levels again? We are tank hunting... :)