Our tanks naturally increase in TDS unless we have denitrification in place, The reason is that when it comes to conductivity/TDS, ions contribute. Nitrate is an ion 
Ammonium is an ion " Water, or H2O, is composed of hydrogen and oxygen atoms. Neutral water contains equal amounts of hydrogen ions (H+) and hydroxide ions (OH-) and is given a pH value of 7.0"
Pretty much anything whose chekical formula contains a + or a - in it is an ion. So, one needs to be careful when using TDS measurements to make decision. All we know from the measurement is the total of everything in the water but not what things nor how much of them.
So if one naturally has nitrate levels rising between water changes, this is increasing TDS. If one's pH drops of rises, this changes the amount of ions in the water. Also, during the time between our water changes the level of organic waste in the water tends to rise. And organics contribute to TDS.
My point is that not all rises in TDS are a problem as much as a natural occurance which we normally handle with water changes. But it also matters that we have some understanding of the sort of things that can cause TDS to go up or down.
Here is an example from my systems. I normally have to set up a bio-farm to cycle filters so I can use them in summer tanks or weekend fish events where I will be selling. It is easier to cyle filters than tanks. In cycling the filters I need to monitor several things. The most important are the carbonates/bicarbonates but also the nitrates. I do not test for these things specifically. Instead I can deduce any problems by know how fast the cycle is moving. When it slows it usually means there is insufficient inorganic carbon for the bacteria and I increase how much crushed coral I have in the tank. I can tell I need to do this because I can see the cycle slowing from ammonia tests and then can tell from the drop in TDS it is likely duie to needing more crushed coral.
On the other hand if the ammonia is dropping but the tds are rising, it means I need to change water due to the build up of nitrate. I do not need to test for it, I just know from the TDS it has to be what has risen.
I use the TDS in a defined situation where I have a decent idea of what to expect. But in a more general situation like a going aquarium, it can be a lot more difficult to determine the exact causes of changes in TDS. There are too many things which contribute to this measurement and for which we do not have test kits. A perfect example would be common salt.
Salt does not show up in GH or KH test, but it shows up in TDS. So if one has added a bit of salt to a tank, they have also changed the TDS. Of course we can test for the specific gravity to indicate salinity levels. But how many of us who keep only FW tanks have this piece of equipment?
Ammonium is an ion " Water, or H2O, is composed of hydrogen and oxygen atoms. Neutral water contains equal amounts of hydrogen ions (H+) and hydroxide ions (OH-) and is given a pH value of 7.0" Pretty much anything whose chekical formula contains a + or a - in it is an ion. So, one needs to be careful when using TDS measurements to make decision. All we know from the measurement is the total of everything in the water but not what things nor how much of them.
So if one naturally has nitrate levels rising between water changes, this is increasing TDS. If one's pH drops of rises, this changes the amount of ions in the water. Also, during the time between our water changes the level of organic waste in the water tends to rise. And organics contribute to TDS.
My point is that not all rises in TDS are a problem as much as a natural occurance which we normally handle with water changes. But it also matters that we have some understanding of the sort of things that can cause TDS to go up or down.
Here is an example from my systems. I normally have to set up a bio-farm to cycle filters so I can use them in summer tanks or weekend fish events where I will be selling. It is easier to cyle filters than tanks. In cycling the filters I need to monitor several things. The most important are the carbonates/bicarbonates but also the nitrates. I do not test for these things specifically. Instead I can deduce any problems by know how fast the cycle is moving. When it slows it usually means there is insufficient inorganic carbon for the bacteria and I increase how much crushed coral I have in the tank. I can tell I need to do this because I can see the cycle slowing from ammonia tests and then can tell from the drop in TDS it is likely duie to needing more crushed coral.
On the other hand if the ammonia is dropping but the tds are rising, it means I need to change water due to the build up of nitrate. I do not need to test for it, I just know from the TDS it has to be what has risen.
I use the TDS in a defined situation where I have a decent idea of what to expect. But in a more general situation like a going aquarium, it can be a lot more difficult to determine the exact causes of changes in TDS. There are too many things which contribute to this measurement and for which we do not have test kits. A perfect example would be common salt.
Salt does not show up in GH or KH test, but it shows up in TDS. So if one has added a bit of salt to a tank, they have also changed the TDS. Of course we can test for the specific gravity to indicate salinity levels. But how many of us who keep only FW tanks have this piece of equipment?
