Goldfish tank problems

Question
Hello,
I have a 10 gallon tank with:
1.) two goldfish, one 2 inch black moore, and one
2 1/2 inch white oranda goldfish.
3.) 1 H.O.B power filter.
4.) about half an inch of gravel, and many hiding
places.
5.) The tank has been cycled and been up and running with the two goldfish for several months.
I know you'll probably say that you should have one goldfish per ten gallons but I am very experienced with fish keeping having in all 7 fish tanks and about 15 bettas. I clean the goldfish's water twice a week with about 20-30% water changes and the filters are regulary cleaned in old tank water. Well, to get to my point, the goldfish are healthy and always active
and looking for food. But, lining the whole top of the tank of the suface of the water is foamy bubbles what is the cause for this? I am very careful about feeding
and I never skip a maintenance appointment. I have done nothing differant to the tank.
To help out:
Ammonia: 0.0ppm
Ph: I haven't tested the PH in a long time and now that I have it seems to be extremely low! Normally it is 7.4-7.6 now it is reading at 7.2-7.0! I do lots of water changes what is wrong? could this be the cause?
I don't mess around with the PH ever and all I put in the water is Aqua plus tap water conditioner to remove chlorine and chloramine.
sorry I don't have Nitrite or Nitrate tests.
Thanks!
Susan.

Answer
Hi Susan;

7.0 to 7.2 is just fine. If it were down to 6.0 I would say that is extremely low! The fluctuations can be just from the seasonal changes that naturally occur in tap water. Mine fluctuates from 7.4 in the summer to 7.1 in the winter.

Usually bubbles that tend to stay on top is from excess ammonia or nitrite. Sometimes it is from water conditioners too. If all the levels seem fine, I wouldn't worry about it. Goldfish also make more body slime than other fish do. That may be it right there. You are doing everything possible to keep them healthy. They are just messy fellas.

Keep up the good work!

Followups welcome.

At Your Service;
Chris Robbins