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Long story short, due to bad advice, about a month ago I ended up doing a water change with dechlorinated tap water instead of (unmineralized) RO water, which I had been using for every previous water change. At that point (end of July) the tank was 1-2 months old. Since I've switched back to tap water, it's been almost another month, and I've been doing weekly water changes, meaning I've probably done 3-4 since I switched to tap.

My tap water is hard, with a pH of 7.2, KH of 89.5ppm, GH of 107.4ppm. As mentioned, I never remineralized the RO water when I first started, because I was dumb. But anyway, I would've assumed my aquarium's water would be similar to that of my tap.

However, after testing using API liquid tests, I have come out with the following parameters

Ammonia 0
Nitrites 0
Nitrates 10 ppm
pH 6.8
Phosphates (checked bc I have green spot algae) 0
KH 71.6
GH 71.6

That means my tank water is way softer than I thought it would be at this point! I'm a bit lost as to how I should go on from here. I have about 5 gallons of RO water (no minerals) from my LFS, and ofc a lot of tap water. I'm wondering how I should go about this week's water change? I want to keep my aquarium on the softer side, for the sake of my plants. I also don't want to totally shock my livestock with a big change (5 frogs, 3 otos, 3 amanos, snails).

But basically, if I continue w/my normal water changes, I'd be foreseeably altering the water chemistry w/every change, until it gets to the parameters of my tap water.

How should I mix my RO & tap to keep a steady pH, when the difference is so weird? I'm not sure how to measure it or even start doing it! And my tank needs a water change! For note, my aquarium is 15 gallons, and I'd say I change the water about 25%-30% of the water every week.

I also, as mentioned, do not want to harm or shock my animals.

If anyone could help with this scenario, I'd really appreciate it.
 

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Ooooh, math. I can help with math. Also I can relate to the using RO water without remineralizers really hard, I did that and kept wondering why my shrimp were dying when I was first starting out with shrimp and RO water...

Let's start with why your tank's numbers are where they're at right now. Remember, RODI water should be at around 7.0 pH or a little bit less than that due to uptake of CO2 from the atmosphere. GH/KH of RODI water that hasn't been premineralized is, for all intents and purposes, 0. You've done about 3-4 weekly water changes using your tap water (GH 107.4, KH 89.5), into a tank that's previously been filled with RODI water (GH 0, KH 0). You change about 25-30% of your 15 gallon tank's water every week. For simplicity's sake, let's say you do 25% water changes. The formula we'll use for both KH/GH is as follows, using GH as an example:

((GH value of new water)(amount of new water) + (GH value of current water)(tank size - amount of new water)) / (tank size)
Let's do the first calculation together:
(107.4)(.25*15) + (0)(15-.25*15) / 15 = 26.85

We'll repeat this over the course of 2-3 more water changes to find the next values of: 46.9875, 62.090625, and 73.41796875, which is close enough to your tank's measured value of 71.6 to account for randomness/external variables that we failed to account for. The same can be applied to KH as well. You can use WolframAlpha to calculate stuff like this easily.

Now that the math is over, the first thing you need to do, is figure out what you want your tank's parameters to be, and then work towards that. So give us a goal of what you want, and then work towards mixing your water towards that goal. Keep doing the 25-30% water changes, since those are less inclined to harm your fish too much.

Oh, and as for pH, that's dependent on the KH. You could choose to worry about it, yes. But that's a pH difference of .4 if you do a tap water water change, and maaaaaybe a .8 if you do an all RO water water change. If it was a pH change of 1.5 units or something, then you'd probably need to bust out a calculator, but for now, don't sweat it too much.
 

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Ooooh, math. I can help with math. Also I can relate to the using RO water without remineralizers really hard, I did that and kept wondering why my shrimp were dying when I was first starting out with shrimp and RO water...

Let's start with why your tank's numbers are where they're at right now. Remember, RODI water should be at around 7.0 pH or a little bit less than that due to uptake of CO2 from the atmosphere. GH/KH of RODI water that hasn't been premineralized is, for all intents and purposes, 0. You've done about 3-4 weekly water changes using your tap water (GH 107.4, KH 89.5), into a tank that's previously been filled with RODI water (GH 0, KH 0). You change about 25-30% of your 15 gallon tank's water every week. For simplicity's sake, let's say you do 25% water changes. The formula we'll use for both KH/GH is as follows, using GH as an example:

((GH value of new water)(amount of new water) + (GH value of current water)(tank size - amount of new water)) / (tank size)
Let's do the first calculation together:
(107.4)(.25*15) + (0)(15-.25*15) / 15 = 26.85

We'll repeat this over the course of 2-3 more water changes to find the next values of: 46.9875, 62.090625, and 73.41796875, which is close enough to your tank's measured value of 71.6 to account for randomness/external variables that we failed to account for. The same can be applied to KH as well. You can use WolframAlpha to calculate stuff like this easily.

Now that the math is over, the first thing you need to do, is figure out what you want your tank's parameters to be, and then work towards that. So give us a goal of what you want, and then work towards mixing your water towards that goal. Keep doing the 25-30% water changes, since those are less inclined to harm your fish too much.

Oh, and as for pH, that's dependent on the KH. You could choose to worry about it, yes. But that's a pH difference of .4 if you do a tap water water change, and maaaaaybe a .8 if you do an all RO water water change. If it was a pH change of 1.5 units or something, then you'd probably need to bust out a calculator, but for now, don't sweat it too much.
That's super helpful! Thank you so much for explaining the math and reasoning behind it all.
I am fine with where my GH/KH is, because it's not too soft or hard, at least based on my knowledge. I'm a tad worried about pH because if I chose to add CO2, wouldn't that drop my pH by about 1 (if I did it right) and make it 5.8? That seems awfully low, frogs are supposed to be in pH of 6.5-7.5, ideally. There's probably a little flexibility, but still.

If I shouldn't worry about my pH, and just wanted to keep everything around that level, would you mind explaining how I could do that?

If there's a way to safely raise my pH while keeping the hardness at a mid level, that would be great to know.

Again, thank you so much!
 

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I have 0 idea about CO2. I like math, and low tech planted tanks, so I can help with calculations, but CO2 dosing is beyond me haha.

If you want to keep the hardness at where your tank is at, all you have to do is figure out what ratio you need to add RO/DI water to tap water. Let's see, so first we calculate how much water you're removing from the tank, which is .25*15 = 3.75 gallons. We can then set up two equations, one for total water volume, and another for water hardness. Let X be the amount of RO water you're adding, and Y be the amount of tap water you're adding.

X + Y = 3.75 gallons

For water hardness, we just multiply the values that we'll need. In this case, we know that the GH of RO water is 0, and the GH of the tap water is 107.4 ppm. If you want to keep the water at 71.6 ppm, we use the following equation:
(0)(X) + (107.4)Y = 71.6(3.75)

As for raising pH while keeping the hardness (GH) at mid level, you could always use something like Seachem pH buffers, which will raise the alkalinity and pH, but not the GH (I think).
 

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Discussion Starter #5
I have 0 idea about CO2. I like math, and low tech planted tanks, so I can help with calculations, but CO2 dosing is beyond me haha.

If you want to keep the hardness at where your tank is at, all you have to do is figure out what ratio you need to add RO/DI water to tap water. Let's see, so first we calculate how much water you're removing from the tank, which is .25*15 = 3.75 gallons. We can then set up two equations, one for total water volume, and another for water hardness. Let X be the amount of RO water you're adding, and Y be the amount of tap water you're adding.

X + Y = 3.75 gallons

For water hardness, we just multiply the values that we'll need. In this case, we know that the GH of RO water is 0, and the GH of the tap water is 107.4 ppm. If you want to keep the water at 71.6 ppm, we use the following equation:
(0)(X) + (107.4)Y = 71.6(3.75)

As for raising pH while keeping the hardness (GH) at mid level, you could always use something like Seachem pH buffers, which will raise the alkalinity and pH, but not the GH (I think).
Your explanations of the math are so helpful! I'm sorry if I bother you with all my questions btw. Do you happen to know how GH and KH are related? If not, no worries!

Also no worries about the CO2. I'll figure that out when I can actually afford it
 
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