Testing water, using RO/DI for FW aquariums

[wonderloss]

What is the best test kit to measure hardness in freshwater?

My wife is interested in setting up a FW tank, and I am going to use my old 55 gallon. She is interested in soft water fish, such as rams. Living in Florida, I assume my tap water is hard. However, I have the RO/DI system for the SW aquarium.

What are the issues with using RO/DI water? Do I need to add minerals to make the water fish ready? If so, what is recommended?

[tippMANn98]

i would say RED SEA's hardness test...

[RyanG]

RO/DI water is too "pure" for freshwater fish and they just dont do well in it. If you run it just RO without the DI membrane that would be fine.

[wonderloss]

Thanks for the help guys.

I am getting ready to shop for some supplies today.

[Boomer]

I would not use and hardness test kit but get a TDS meter which can be used for water hardness and how well you your RO/DI unit is runniung if you get one. They range form ~$20 - ~$100.

The Filter Guys

[wonderloss]

That sounds like a good idea, Boomer.

I have an inline TDS meter on my RO/DI. It reads after the RO and after the DI. I do not have a handheld meter.

Can you point me to some good articles on TDS, hardness, etc? I specifically would like to know how to convert TDS to a measure of hardness, but I like the more thorough background knowledge too.

[Boomer]

I have an inline TDS meter on my RO/DI. It reads after the RO and after the DI. I do not have a handheld meter.

A good way to go.

Can you point me to some good articles on TDS, hardness, etc? I specifically would like to know how to convert TDS to a measure of hardness, but I like the more thorough background knowledge too.


A General Hardness test kit is measuring the amount of Calcium and Magnesium in the water. However, most of the hardness in FW is a function of the Calcium. Technically speaking, it is really the sum total of Calcium, Magnesium, Strontium and Lithium. The later two are so small they make up less that 1 % in FW of the Total Hardness, so are not an issue.

TDS is similar for FW. However, it is the total of all the ions in water that conduct electricity, so would also be ions like Sodium, Chloride, Sulfate, Nitrate etc.. In FW tanks one does not really need to know what the Calcium level is, it is the total ions you need to be concerned about. In tap water, streams and lake waters Hardness is mostly Calcium i.e., >75 % TDS.

The only issue with Hardness vs TDS, is if one is adding salt to their FW tank, which many do. This will not raise the GH but will really raise the TDS. If you have a need to know the Calcium level then by all means also have a GH test kit. I have yet seen a need for a GH kit and I raised both FW and SW fish for almost 35 years.

TDS meters are often used for Hardness, although it is really not an accurate means. The real reason behind hardness and TDS is to determine the amount of Calcium in the water, as Calcium causes hard water deposits. This is a concern in the water /wastewater industry and is really where both TDS and GH came form. Most water/wastewater companies no longer use GH kits but TDS meters.

convert TDS to a measure of hardness

You really can't get a real conversion just one close enough and it also depends on calibration solution the TDS meter is calibrated with. The 442 solution is the best for FW and you can read more about it here.

What is TDS?
What is TDS? by Randy Holmes-Farley - Reefkeeping.com

UNDERSTANDING WATER HARDNESS
WATER HARDNESS -- CALCIUM &* MAGNESIUM

[Coralreef]

I have freshwater with tetras and plants and saltwater reef. I also have a RO/DI that I use to fill and do water changes on both. It works just fine. TDS are 0