Your tank would not be considered overstocked but I would say that it's near its stocking limit.
The general rule of thumb is 10 gallons of water per each goldfish. You're tank is fine in this aspect. Tetras usually do not grow too big so they do not present much of a problem but the pleco can and will grow to 8+ inches which in the future might put your tank over the limit in terms of stocking.
Now there is something to keep in mind here and that is that goldfish are not tropical fish and thus do not require the warmer water temperature that tropical fish need so it's important that the water temperature be kept in a range of 68f-72f degrees.
How long has your tank been up and running with fish in it?
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Ok that's great. I just wanted to make sure this wasn't a new set-up because that would put a whole new different light on things.
I'd like to compliment you on your selection of fish as it's a very interesting and unique combination particularily with the shubunkins and tetras. You did well by checking the ammonia level and should keep doing so every week as sometimes the addition of new fish will cause a temporary ammonia spike.
I would recommend that in addition to testing for ammonia you also test for levels of
nitrites and nitrates. Ideal results should always indicate ammonia and nitrites=-0- and nitrates 20ppm or under. Ammonia and nitrites should never be present in an established aquarium as they are both harmful and often lethal to fish.
Now the only thing that has me concerned is the ph of 6.0. This ph is fine for tetras but not for the shubunkins nor the pleco. The first thing to keep in check is that the ph does not go any lower or crash as this could present a problem.
In a tank such as yours an ideal ph would be 7.0 but raising it to this level from 6.0 would be quite a chore. Rather than recommend using additives to raise the ph what I suggest you do is add some crushed coral. It will naturally raise the ph and does not need to be changed. You can use the crushed coral as a substrate (gravel). It can be found at most pet stores where gravel is sold. I have used it and still use it in my livebearer tanks with great success.
I would start out by adding 5 pounds of crushed coral and then test the ph to see if the crushed coral has had a significant effect. If not then add more. The only foreseeable problem with using crushed coral is an esthetic one. If you're current gravel is of a darker shade you might find that the crushed coral clashes with it as it's light in color.
Carole please keep in mind that adjusting ph can be difficult and any changes in ph should be done gradually as what appears as a small movement in ph, i.e. 6.6-7.0 is actually a very big change for fish.
Hope this information helps and please let me know if you have any questions.
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