Shimmies as a sign that something is wrong are usually due to water quality issues. Usually this relates to the parameters but can also be due to water quality. It can be related to the electrolytes in the water, usually something is missing. You did not report either your GH or KH numbers which very well could be relevant here.
What I am seeing in your post is you are keeping fish which require different parameters. The odd things here is many tetras like softer water which is also usually a lower pH. When you say bottom feeders this is about as little info as you might offer on fish. Corys? Plecos? Loaches? I could go on, but it isn't likely you have more exotic ones. It can also matter whether the fish are wild or farmed. Normally, wild fish are more sensitive than tank or farm raised. So, your tetra may have adapted to the water parameters and the change is what caused the problem. I cannot say this is what happen, however. 7.6 ph is not bad for farmed tetras. The worst it might do is to prevent successful spawning.
High nitrates are not good. As FF say- weekly water changes of 50% should normally be done in almost all tanks.
Now to make things more complicated, there are internal parasites that can cause a fish to have the shimmies. This would be much harder to determine. The parasites take nutrients from the fish and this results in imbalances. Also parasites can get into a brain and really mess up things.
However, it is usually best to start from the most likely causes and then work towards the rarer ones when trying to diagnose things. If you start to have a cough do you think a cold or flu or lung cancer as your fist thought? It is the same for fish. Eliminate the obvious before looking for the rare.
