help! beginner and worried!

Question
Hello again Karen
Thank you so much for all your help so far. I think it has helped us understand  how to look after our fish better.
just to update you on what we have done so far.
our first change of water we didnt dechlorinze (at time didnt know how to )  and we took the following water tests a day or two after;
nitrate no 3 read 20 ppm, ph 7.6,ammonia .25
nitrite 0.25 these taken on the  27/5/06
we changed the water  on the  4/6/06 and addede some tetraqua aqua safe conditioner. that apparently makes tap water safer! neutralises chlorine and heavy metals harmful to fish. protects against wounds etc and reduces stress.
and cleaned with a magnet scraper  (thanks for advice on them i neded up getting a magnet one and a scourer type one. magnet one doesnt seem to stress fish.
the glass looks so much cleaner and so does the plants and gravel as we also used s gravel siphon cleaner.
i hae just performed the next set of test having left the water change to settle (is that right) and the readings are;
nitrate 3 reaads  20-40 (i am not sure as really is between the two)
ph is  7.6
ammonia is 0
ad nitrite is  0

both the silver dollar and the mystery fish look well and eating .
when woudl be good time to introduce new members to the family?  and how best to do this?
and thanks for  your link . yes the mystrery fish looks soo much like the black tetra.  more like the fourth picture on your link.
well done on your guess. you know your fishes!!!
I have passed on your best wishes to the two survivors and they said thank you for saving us.
you have been a great help Karen.
what fish do you recommend mixing with these two/ also we have a 48l tank so how many fish can we get?
Thanks again so much we both are enjoying 'fishing' now
Caren x  

Answer
Dear Caren,
Although I cannot say for sure 100%. It sounds like your aquarium is near cycling completely! The water change probably diluted the ammonia and nitrite levels down to zero so just to be on the safe side, I would continue testing your aquarium water everyday for at least the next 4 days and see if any ammonia or nitrite pop up. If they do, simply wait (preform a water change should the readings be out of the safe zone) and let the aquarium finish cycling. But if it is consistantly Zero of the past several days, I would consider it safe to add a few more fish to your current little fish family.

Knowing how many fish an aquarium can hold is a tricky one. Many petshops tell you with "todays" filters, you can keep any number of fish you want in your tank. This is so far from the truth. Or you may get "one inch per gallon" this is only true for very slim bodied fish like Zebra danios or Neon tetras. Thicker bodied fish even though they may be one or two inches will always produce a higher bioload than the little slim bodied neons or danios. I know a very reliable website that has a good calculator for small community fish aquariums. It combines the dimensions of your tank and gives a general guideline to the capactity of the aquarium- Here's the link-
"www.fishtanksandponds.com/resources/calculators/stockingcalculatior.htm"

As for fish mixing. Well. Your Silver dollar is probably quite lonely in his aquarium. They are in fact a schooling tetra and yours would appreciate a few more friends. Although your aquarium size is roughly 12 gallons (48L) this is really too small for a group of Silver dollars. It can be suitable for a while until the Silver dollars grow larger. If the mystery fish is a black Tetra, they too enjoy being in a group of at least 4 or more. Insure your aquarium is established first before adding anymore little fish or aquatic life first. ;-)

I really hope this helps and it's so good to know your fish and aquarium are doing just excellent!

I'm also glad to know my advice has given you some better insight to fishkeeping and I wish you only the best as always! Feel free to write with anymore concerns!

Happy 'fishing'! ;-)
Karen~