Assuming that it is not due to clorine, or other external medication, water quality is poor due mainly to bio load. Test for NO2 & NO3.

I also agree that a FH is a tough fish. If you don't want to spend too much $$, try doing the following things ...

1. change 50% of the water every day for 3 days in a row. If more, it may stress your fish. By doing so for 3 days in a row, you dilute your original water by 50% during the first change, 25% the second, and 12.5% by the third. This way, you "reset" your water ... thus, no need to test for water!!!

2. put in empty pots, or wood for the fish to hide behind. With more hiding space, the fish will feel more secure, and thus, less likely to hide!!

3. put in ditter fishes. These are FAST swimming fishes, like zebra denion, etc. If the FH sees other fishes swimming in the open, it will feel safer, and tend to venture out in the open.

I believe that the above steps will help de-stress the fish. If not, I suspect sickness. Observe external for any parasites, and its waste and belly for internal parasites.