Fairly new aquarium, fish dying

Question
Hello Karen,

We have had our 25 gallon aquarium set up for about 3 months.  We started with 3 black skirt tetras for about 3 weeks, then added 10 neon tetras, and 1 suckerfish, a rubbernosed something or other.  Over the next 5 days, 5 of the neon tetras died, while the other fish seemed normal.  Over this time period we were testing the water for ammonia, nitrate, nitrite, and ph, as well as approx. 20% water changes each week.  Although the ammonia spiked up for a couple of days to somewhere between .25 and .50 ppm, the water changes seemed to take care of the problem.  We have been adding Prime water conditioner, about 1/2 a capful to each water change.

2 weeks later we bought 3 powder blue gouramis (dwarfs, not 3 spots), 3 fire gouramis (also dwarfs) and 1 otto cat.  Things seemed fine for a couple of weeks, then all of a sudden we lost the largest powder blue.  The day before he died we noticed some kind of white growth on the top of his head, which looked like it was eating into his head, leaving a depression which was white and sort of fluffy looking.

About a week later we lost the second powder blue, and the next day we lost the rubbernosed suckerfish.  Both fish died with no warning, and no symptoms of any disease.

And tonight we just lost the third powder blue.  She developed red sores on her mouth and fins, and was lying sideways on the bottom.  She would swim to the top and suck some air, then sink back to the bottom.

We have been changing water weekly and monitoring the water conditions, and although the nitrate level spiked to 10 ppm the day after the rubbernosed suckerfish died, the ammonia, nitrite, and ph have all remained well within acceptable limits.

As I said, the tank is 25 gallons, with an in gravel filter, no vegetation, and is kept at 25-26 deg. celcius.

Any information you may have as to what is killing our fish, or any suggestions on the aquarium would be most welcome.  
Thanks.

Answer
Hi Ryan,
So sorry that you are having problems with your new aquarium.
It is most likely caused by not being completely cycled. You did good by checking and monitoring the ammonia and nitrite levels and adding the Prime is also a good thing. The neons dying is not really unusual since they are a bit delicate and any fluctuation in water quality will cause problems.

When a tank isn't completely cycled it will always have some ammonia and nirites. Most fish are very much affected by even small amounts. Even a small amount can cause skin and fin damage and sometimes death to some more sensitive fish. Each time more fish are added to a tank, even when it is cycled, there is a larger bio load. The beneficial bacteria have to be able to handle the extra load and if they are not numerous and well established they just can't handle it and the water quality suffers and so do the fish.
Whenever a tank is cycling you can do more frequent water changes than you were doing. It was good that you were doing 20% water changes but you probably should have done them more frequently since the tank wasn't cycled and there was  quite a few fish producing waste and ammonia.
For a tank to be completely cycled the ammonia and nitrite should be at 0. Also, one other thing you might want to really be careful about is the undergravel filter. They are good as long as you can keep any fish waste and decaying food siphoned out of the gravel. They are just a little harder to keep clean and things can get trapped under the plate and cause your water to suffer. It might be easier to maintain if you got a filter that just hangs on the tank and then you can rinse the filter media in aquarium water whenever it gets dirty.

One other thing you might try  to get more bacteria established, is adding Cycle or Bio Spira. They contain some bacteria that will help to get your tank cycled.

I have had great success cycling tanks with just a couple of black skirt tetras. I would do a partial water change several times a week and wait until the tank cycled and slowly add only a very few fish at a time so the bacteria could handle it.

Try giving your tank more time to completely cycle before trying to add anymore fish. Do more frequent water changes to help control any ammonia or nitrites. After cycling be sure to do water changes at least 1-2 times a week just to keep things really clean. Never overfeed fish since this will cause an unset in water quality.

Your fish that died probably had fungus and ammonia caused alot of the other problems. It can really act fast and kill fish.

Just be careful and I think you will get everything under control and have a nice aquarium in a few weeks. Be patient and it will work out.
Hope this has helped,
Karen