A tank that has been going for only a week is likely not cycled yet. With all the fish in there the Ammonia levels should be very high. I would invest in an Ammonia/Nitrite/Nitrate test kit and monitor the water. I would also look into whatever the LFS is selling to help speed the Nitrogen Cycle.

If this had been an established tank, with filter medium that is keeping up with wasteproducts I would suggest raising the temperature to near 80, along with increasing the aeration/filtration. The warmer the fish the more activity ... and metabolism. The lower the temp the less activity.

I took out a tiny bit of water so that the water coming off the filter circulated the water and created more oxygen, for now. (i read that somewhere online) I couldn't get to Petco today. There is a small fish store about 35 min. away but I don't feel I can really go in there and ask for advice since I haven't bought any fish there. Ds was given a $50 gift card to Petco along with the aquarium, so he got the fish at Petco with his gift card. We went to check out the fish store and it was very clean (petco is really dirty) and the girl working was very friendly and helpful. I only bought a new tank cover while we were there. The heater is turned up as high as it will go and the water temp is 78. Is that okay? Any other suggestions until I can get my water tested again? Should I change out some water? They still aren't swimming around much. Everything I've read says that tiger barbs are very active fish (they were the first two days) and they are just sitting there. They did eat when we put in a few flakes earlier.

78 is a good temp. If the water gets cloudt I would do a water change. But that's like a bandaid if you are experiencing "New Tnak Syndrome". Play it by ear and wait until you can get a test kit.

Your best LFS's are the mom and pop places where the owners are true hobbyists. The chainstore places tend to have kids / young adults, who know a little... but not a lot about fish.

All the fish are dead except the rainbow shark. He stays in a little cave and doesn't come out. I bought a test kit and of course everything is very high. I did a 25% water change yesterday and today. Ammonia/Nitrite/Nitrate is still very high. I've been googling, trying to figure out what to do. I've read to do 25% water changes until the levels come down. I read someone else say to do that twice a day. I really want this shark to live since my kids are both so sad over all the rest of them dying.

I would do a water change if the tank is cloudy and just wait the cycle out. Obviously, now isn't the time to add new fish. Sorry that you and your family had to go through this. I wish the people at petshops would take a proactive approach to educating new customers about the Nitrogen Cycle, instead of trying to make a quick buck.

So doing water changes to help the shark stay alive won't work?

thank you petiolaris for your help.

Now the shark is in this tiny little cave inside what is supposed to look like an old broken down castle. He's inside the castle. I can't believe he fits in there, the door is so tiny. I can barely see him in there. He moves once in awhile and I can see him. He came out earlier when my dd turned on the light. We turned the light back off and he went back in there.

I've done 25% water changes yesterday and the day before. My levels are still very high. I'm wondering if I do a 50% water change today if that will help him out. It couldn't hurt him could it?

Is the tank still cloudy? If not, I wouldn't do any water changes. Sharks tend to be less tolerant of major water changes, relative to other species. If he's hanging in there, the less tinkering... the better.

You have to put a water conditioner in your tank. I recommend Tetra. It corrects the PH levels, nitrate levels, and takes out chlorine. So your fish will survive