While it is a good thing to have the TDS as supplied by the water comany, but I would make two observations about this. Reports are usually based on a some number of individual tests or perhaps a single one. However, given all of the variables involved with public water supply, including private pies not under regulation the way public pies are, one needs to be aware that what comes out of their tap may not have the exact specs their water company supplies. What is more important is to test what actually comes out of their specific tap.What is most important is what is the TDS in any given tank.
Why this is the case has to do with how TDS is calculated. Al the meters used actually are testing for conductivity. Contrarty to popular belief, pure water is a poor conductor of electricity. What makes most water a good conductor are all the other things in it. These are what actually conduct the electricty. So what a conductivity meter does is to send a small current between two electrodes in the water. They know how much is being sent and what is received determines conductivity.
Units of Measurement
Electrical Conductivity is the ability of a solution to transfer (conduct) electric current. It is the reciprocal of electrical resistivity (ohms). Therefore conductivity is used to measure the concentration of dissolved solids which have been ionized in a polar solution such as water. The unit of measurement commonly used is one millionth of a Siemen per centimeter (micro-Siemens per centimeter or µS/cm). When measuring more concentrated solutions, the units are expressed as milli-Siemens/cm (mS/cm) i.e.- 10-3 S-cm (thousandths of a Siemen). For ease of expression, 1000 µS/cm are equal to 1 mS/cm. Often times conductivity is simply expressed as either micro or milli Siemens. However this unit of measurement is sometimes (incorrectly) referred to as micro-mho's rather than micro-Siemens. The expression "mho" was simply the word ohm spelled backwards.
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http://www.mbhes.com/conductivity_measurement.htmWhat a TDS meter measures is conductivity but then the device uses a preset formula that should also account for temperature, to convert the micro-Siemens into TDS expressed as ppm. For non-scientists like myself, TDS can be a bit easier to understand. The other thing to realizes id TDS or conductivity measure the total of things but they do not tell us what those things are nor hoe much of any them might be. Both GH and KH contribute to TDS, but so also do ions. So nitrate in the water will raise TDS, for example.
While it helps to know one's GH, this does not change due to the presence of salt (sodium chloride). If you measure the TDS of fresh water and then add salt to it, the TDS will rise but the GH will not. It may be helpful to understand this all when one realizes that one measure of both GH and KH is ppm. 1 degree of GH or KH can be converted to ppm by multiplying each degree by 17.8 to get the ppm or vice versa.
I use a TDS meter because I breed some fish which, in the wild, are triggered to spawn due to the seasonal change in their water due to having a dry and a rainy season. One of the things that makes these two seasons different in terms of water parameters is a change in TDS and temperature. Of all of the digital testers oen might want, a TDS meter is the least expensive. Mine was about $22 and reads both TDS and Temp. I also have a continuous 3-way continuous monitor that reads temp, pH and TDS, that cost me well over $200 and that doesn't include the needed calibration solutions. Most fish keepers do not really need this equipment.
Looking at your TDS of 112 ppm, you have soft water. My tap is 83 ppm tops and when we get big rains a for a few days I have seen it drop to 57 ppm, But I have a private well. But my TDS means my water pretty soft. My Gh is about 5 dg and my KH is about 3 dg.
Water hardness follows the following guidelines. The unit dH means "degree hardness'', while ppm means "parts per million'', which is roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm CaCO3. Most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not mean it actually came from CaCO3.
General Hardness 0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)
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http://fins.actwin.com/mirror/begin-chem.html#referenceThis is important because you are keeping a number of fish that prefer harder water and some that prefer softer water. Otos, angels and the bn like softer, but the bn can take harder as can the angel. The rest of your fish want hard water and need it for long term health.
Given all of the above I am sure you have multiple issues. One is the problem with farmed guppies and the other is due to your water parameters. It may also be they there is a pathogen in the tank that could have come in with any of the fish. Bear in mind that different fish are more or less susceptible to diseases and parasites, A fish that survives ich build up an immunity to it which will last some number of months is an example. And then stress will always weaken the immune system of a fish.
Sorry to be long winded here, but I hope it helps you see the bigger picture. I think a few things would be helpful for you to consider. The first is quarantine for new fish prevents a lot of problems. The second is that you rethink your stocking some. If you want to keep fish happier in soft water do that. if you want fish that are happier in harder water, then you will need to use an additive to maintain that hardness. Increasing hardness is easier to do in a stable fashion than the reverse.
