For my new build I am using conductivity or ec, a reliable method of measuring tds as long as you know the conversion factor for your meter (the Hanna ďdistĒ series uses a .5 conversion, so a measurement of 200 microsiemens on the meter represents a tds of 100 ppm), as the guiding metric for when I need to dose ferts, perform water changes, etc..
Iíve found ec/tds to be the most reliable and simple way to determine when adjustments to the water are necessary, and would definitely recommend it to anyone looking to fine tune their dosing/water change routine without having to test a million different parameters. It works whether you are using tap water or ro/di (although I think itís an even better indicator when using ro/di because you can control the exact make up of the tds in your tank).
With tap water (and rodi for that matter) you need to establish two baseline tds readings in order to use it as a determinitive metric- source water and baseline tank water. Letís say your water comes out the tap with a tds of 150ppm (or an ec of 300 microsiemens), and the water in your tank before ever dosing ferts or immediately after a 50% wc has a tds of 250ppm due to fish waste, nutrients released by substrate/driftwood/decaying plant matter.
The 250ppm number is the baseline in this example. If youíre using a dosing method like pps or pps pro- you would add ferts at the suggested rates until you observe a 100ppm increase in your baseline tds (350ppm in this example). Once your tds has increased by 100ppm, it is now time to do a water change to reset the tank back to its baseline of 250 (or as close as you can get).
This is where using tap water can get tricky- if you are replacing the 350ppm water with tap water that has a tds of 150ppm, you will need to change far more water to get back to your baseline than you would if you were using ultra low tds rodi water. My rodi unit puts out water with a tds of 1-2ppm which is exceptionally low for a home purification system. I wonít get in to the math, but one can clearly see that using 2ppm water for a change will be much more efficient at getting you back to your baseline than if you use water thatís 150ppm. You also have the benefit of knowing exactly what elements are responsible for a given tds reading when starting from 0 (or close to it) because you are adding them in. With tap water, the tds could be the result of any one or a combination of a number of elements that we cannot control for (in my town we have pretty high NACL and iron content in our tap, so those three elements are the main sources of tds but there are certainly others like calcium, magnesium, silica, etc. that also contribute. I strongly suggest going on your towns website and accessing your most recent water quality report to get an idea of whatís in your tap water).
Most of the elements that would raise tds in your tap water are beneficial nutrients for plants, but itís impossible to determine the concentrations of each without expernsive testing or very detailed and accurate town water reports. Again, this is why I prefer rodi water when using ec or tds as the determining factor. However, as long as you have a baseline to work from and your tap tds isnít higher than your target baseline tank tds, tap water or a mix of tap and rodi can work.
One last note on baseline tds- Iíve seen a lot of people both here and on other sites claim that tds values in and of themselves are worthless measurements- this may be true in a tank with no fish, but if you do keep fish in your fish tank lol the claim that tds is a worthless measurement is completely incorrect and I will gladly argue that point with anyone who wishes to do so.
Most of the fish that we commonly stock in our planted tanks (most tetras, rasboras, gouramis, non-rift lake cichlids, etc.) have evolved to live in low tds conditions and rely on the relatively pure water in which they live to remove contaminants from their bodies. A classic softwater tetra like a neon or cardinal will producd as much as 3.5 times their body weight in urine every day. They take in the ultra low tds water from their surroundings and use it to flush nitrates and other nutrients from their bodies. This works because they are adapted to the relative osmotic pressure of the low tds eater in which they live. If you place these fish in conditions where the tds is approaching the levels inside their bodies, their internal organs must work much harder to purge the nutrients from their bodies which places undue strain on their kidneys and liver, which in turn stresses the fish and weakens their immune system, shortening their lifespan and making them far more succeptible to disease.
Some Amazon biotopes have natural tds levels as low as 10-20ppm! Aquarists looking to succefully keep and or breed fish from such regions almost always rely on a rodi system for their water. Even then, levels this low are not really attainable in a planted tank setting, and would be problematic for the plants. You also have to make sure you provide enough mineral content to allow the fish to perform their metabolic functions.
Because I keep tetras from very low tds environments, I try to keep my baseline tds at around 120-150ppm and will change the water once it hits 200ppm. This is just the number Ive settled on based on what works for my fish and plants but everyone has to come up with a number that works for their set up. Observation of plant and fish health is the best way to make that determination.
There are plenty of people who keep tetras like rummy nose, cardinals, neons, etc. with tds thatís double mine, but as previously suggested, I would consider 500ppm to be the absolute cap on what constitutes a healthy low tds environment for Amazonian species and personally would not want to exceed 300ppm.
Many issues with fish that get attributed to ph swings or hardness are actually caused by osmotic shock, which can occur when fish are subjected to a large swing in osmotic pressure, measured by tds/ec. So contrary to a lot of the info thatís out there I believe tds/ec May be the single most important water parameter we can measure. If you are working with rodi water or known quantities of nutrients in your tap water, no other measurement will be as instructive for dosing and water changes as tds/ec. Period full stop.
Experience: worked in fishroom of Eddies Aquarium in Albany all through college, 20 years of keeping reef tanks, planted tanks, and dart frog vivariums.
Current/in-progress tanks: 20 gallon hi tech tank in office, 120 gallon nature/tetra aquarium at home.
Philosophy: work with what youíve got, and make choices based on the conditions that easiest for you to maintain over time.