Disease diagnosis is a very difficult topic in this hobby. But reading through this thread initially and then agai9n just now, I see I missed something in post #1 that may well be behind all this. And that is the GH.
Before that, nitrate is also a serious issue here. As members have correctly been mentioning, you want to keep nitrate below 20 ppm, and as low as possible beyond this. Nitrate is detrimental to aquarium fish, it just takes longer than it would for ammonia or nitrite. And the best way to explain what it does is to say that is weakens the fish. This in itself has ramifications: fish have more difficulty carrying out normal every-day functions which debilitates them even further; it causes stress and stress weakens the immune system and is the direct reason for 95% of all fish disease problems; it shortens their lifespan (because of the weakening and stress). There are a few studies now that seem to suggest nitrates are responsible for other issues, especially in cichlids. So, it is always wisest and safest to keep nitrate as low as possible, and never over 20 ppm.
To the GH. If the GH is closer to the 100 ppm end, this is not sufficient for livebearers like swordtails. These fish must have moderately hard or harder water, because they are designed to make use of the minerals calcium and magnesium in order to carry out normal functions of their physiology. This also weakens them. The ph is related, it must be above 7.0 for these fish. The GH should be at minimum 10 dGH (= 180 ppm) but above this is preferable.
The other mentioned fish like neon tetras are soft water species so that is why they would not be showing the symptoms the swordtails have been. And this fact would tend to suggest that the GH is the actual issue here, though that does not rule out other sources. But the fish are being weakened by both the GH and nitrates, so those are the prime causes.
You need to decide on soft water species going forward, or on raising the GH/pH for the livebearers...but keep in mind that the neons will not appreciate that. Given your source water, the easiest solution is to not have livebearers but only soft water species. Or set up a tank just for livebearers with a calcareous substrate that will permanently dissolve and raise GH/pH. Swordtails get largish and need a 3-foot tasnk, if that is what you decide.
The nitrate must still be brought under control. It is likely this was caused by the organic load, especially from a 9+ inch pleco as these are notorious as waste producers. Removing the pleco as you intend, and doing a good clean of the substrate, keeping the filter well rinsed, and significant water changes while not overfeeding and not overloading the fish stock should keep these within reason (below 20 ppm, below 10 ppm preferably). Some of my tanks are fairly well stocked but nitrate remains in the 0 to 5 ppm range and has for years now. Live plants especially floating species will also help.
Before that, nitrate is also a serious issue here. As members have correctly been mentioning, you want to keep nitrate below 20 ppm, and as low as possible beyond this. Nitrate is detrimental to aquarium fish, it just takes longer than it would for ammonia or nitrite. And the best way to explain what it does is to say that is weakens the fish. This in itself has ramifications: fish have more difficulty carrying out normal every-day functions which debilitates them even further; it causes stress and stress weakens the immune system and is the direct reason for 95% of all fish disease problems; it shortens their lifespan (because of the weakening and stress). There are a few studies now that seem to suggest nitrates are responsible for other issues, especially in cichlids. So, it is always wisest and safest to keep nitrate as low as possible, and never over 20 ppm.
To the GH. If the GH is closer to the 100 ppm end, this is not sufficient for livebearers like swordtails. These fish must have moderately hard or harder water, because they are designed to make use of the minerals calcium and magnesium in order to carry out normal functions of their physiology. This also weakens them. The ph is related, it must be above 7.0 for these fish. The GH should be at minimum 10 dGH (= 180 ppm) but above this is preferable.
The other mentioned fish like neon tetras are soft water species so that is why they would not be showing the symptoms the swordtails have been. And this fact would tend to suggest that the GH is the actual issue here, though that does not rule out other sources. But the fish are being weakened by both the GH and nitrates, so those are the prime causes.
You need to decide on soft water species going forward, or on raising the GH/pH for the livebearers...but keep in mind that the neons will not appreciate that. Given your source water, the easiest solution is to not have livebearers but only soft water species. Or set up a tank just for livebearers with a calcareous substrate that will permanently dissolve and raise GH/pH. Swordtails get largish and need a 3-foot tasnk, if that is what you decide.
The nitrate must still be brought under control. It is likely this was caused by the organic load, especially from a 9+ inch pleco as these are notorious as waste producers. Removing the pleco as you intend, and doing a good clean of the substrate, keeping the filter well rinsed, and significant water changes while not overfeeding and not overloading the fish stock should keep these within reason (below 20 ppm, below 10 ppm preferably). Some of my tanks are fairly well stocked but nitrate remains in the 0 to 5 ppm range and has for years now. Live plants especially floating species will also help.
