Perhaps your tank was balancing itself in a cruel way. Regardless the tank stocking level is good now and I wouldn't add anymore fish. I think the problem is either an illness introduced with the cardinals or else the massive shock to the bioload. 10 fish all at once is hard.
Nitrates are a good indication of when water changes should be done. Obviously in a planted tank nitrates are getting adsorbed by the plants to some degree but other harmful elements are not. Some when the nitrates are hitting 40 ppm in a planted tank the other elements might already be getting quite high.
Generally a weekly water change of 30-50% in a 10 gallon tank will help to keep levels safe with you current bioload.
Nitrates are a good indication of when water changes should be done. Obviously in a planted tank nitrates are getting adsorbed by the plants to some degree but other harmful elements are not. Some when the nitrates are hitting 40 ppm in a planted tank the other elements might already be getting quite high.
Generally a weekly water change of 30-50% in a 10 gallon tank will help to keep levels safe with you current bioload.
