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Type of water to use in a freshwater aquarium


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Hello all!  My son and I forayed into this world of aquarium keeping in June of this year.  We set up a 15 gallon columnar tank with a HOB filter (with a surface skimmer we added later),  preset heater, Niko rock for caves, Fluval stratum substrate, several types of plants.  We currently have 7 harlequin rasboras, 3 cherry shrimp, three Nerite snails, and two river snails that came in on some duckweed we tried (we’ve since removed the duckweed as it was complicated to deal with while using the skimmer).  We’ve been learning as we go and have made some mistakes along the way.  However, we’ve also had some successes.  

I have been wondering what my best option for water in the health of our tank and it’s flora and fauna might be.  We are on softened well water but, have access to   RO water (we have two gallon tanks and a pumped system).  I’ve been using the RO water in the aquarium since I figured the salts in the softened water would not be good for the plants.  It is somewhat of a process to do water changes since I have to save up water if we need to do a water change of any substance.  My understanding is RO water has minerals and elements removed from it, which may not be optimal for plants or invertebrates.  The well water seems to have a lot of iron in it; I have not measured it in any way, just based on how it stains things orange.  This is not uncommon in northern IN. 

Currently the GH in the tank measures at 5, the KH is at 3, and the pH is 7.0.  From the RO faucet the GH is 1, KH is 1, and pH is 7.6.  From the well (before softening), the KH is 14, the GH is 13, and pH is 7.2. 

My questions are:  Is RO water the best option for use in our aquarium?  Will the lack of hardness in the water adversely affect the invertebrates? (The shrimp seem to be molting and growing so far).  As for pH in the aquarium, it has vacillated between 6.6 (in the very beginning before fish were added) and 7.8.  Most frequently it is around 7.2.  How much pH fluctuation do the inhabitants of the tank tolerate (last week it measured 7.8 and today it is at 7.0)?  The rasboras have been glass surfing a lot and I’m wondering if it’s the pH fluctuations.

We are anxiously looking to add 6 special ordered Corydora Hasbrosus sometime soon.  We want to make sure we are set up for success.

I greatly appreciate all and any input!


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RO and RO/DI is great water for an aquarium, especially for a planted tank or ones with shrimp. Are you using an RO only system (prefilter, carbon blocks, and then RO membranes) or are you going through the whole RO/DI process with additional deionization resins? Depending on which process you use determines how much work you need to put back into the water before using it. If the RO unit is for drinking, I doubt you have a full RO/DI setup as it's not ideal for drinking since your body does need the trace minerals in water. This as it is is fine for aquariums. The thing about it is RO/DI needs to be remineralized. If you do have a DI portion, that's where the minerals are stripped as they are pulled into the resin which are positively or negatively charged in order to produce the purest water possible. 

If you have bad source water, have known issues due to your source water, then RO/DI is the way to go. You control the water with additions of buffers to stabilize the parameters. On top of that you will need to make sure to fertilize with comprehensive or multistage fert methods to get the nutrients back into the water in a controlled manner. I'm also on a well and have a softener unit as well as an RO/DI unit.

If you wish to use tap, you can physically tap a line pre your water softener unit to get water that is non-softened from the source, I've done this for my tap water for my fish room so I can mix this with the hot water which is softened (as I didn't want a second water heater). I also use RO/DI but I save it for tanks that require the extra scrutiny of water parameters. The unit was setup for my reef tanks and stores the product water in a 60 gallon water storage tank that I pull from when I need it. If you plan on going RO or RO/DI you might want to consider setting up a storage tank for your RO/DI to fill which can simply be done by tapping your produce line and splitting it off to a container with a float valve. The container will constantly be filled when the valve is open, but once the water level rises to trigger the float the feed will shut to stop the flow of produced water.

One of the main things I did with a well was put a multi stage sediment filter on my feed as to keep the tannins from clay from getting into my tanks. It's yet another thing to maintain, but it makes a huge difference in clarity. 

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You are right, Tihshho, we do only have a RO unit and not an RO/DI unit.  I had wondered it it might be beneficial to mix well water (pre-softener) with RO water; that ability could be useful, especially if we decided to get more tanks.  I tested the parameters of our well water (drawn from an outdoor spigot) along with the GH and KH.  It showed 0 ammonia, nitrites, and nitrates.  As for any contaminants, I couldn’t say and have thought to take a sample in for testing at the county office.  A larger storage tank for reserve RO water would be luxurious!  

I have recently learned about making sure to provide fertilizers for the plants, thanks to aquarium co-op videos.  We now have easy green and root tabs to work into the regimen.  

I really never realized all that caring for an aquarium entails.  I do enjoy it and it has been fun to get into with my son.  

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With a well one thing to consider is CO2 content from the tap. Some wells, like my current one, have high CO2 out the tap. If you do too large of a water change with it or don't have a bunch of air flowing (sponge filters or airstones) to keep it agitated you can gas your fish and suffocate them. 

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