[mannyaf7]

Hi all, fairly new to the forum, and I have a bit of a conundrum:
Due to a bit of an algae problem, I’ve started checking the water parameters in my tank to get some insight on where the imbalance causing the algae is. I’ve been trying to keep my nitrates between 20-30ppm because I’ve noticed it really makes my plants put out new leaves, and I hope that way they’ll outcompete the algae.
Recently, I’ve noticed something very curious though. I did a 50% water change (my tank is 5 gal) and the nitrates, which were at 20ppm before the change dropped to 1.5ppm after I was done, which confuses me because I thought they ought to be at ~10ppm. I add a bit of liquid fertilizer, wait to the next day, and measure 6.2ppm.
I repeat the experiment but this time with a smaller 20% water change. My nitrates, which were higher than I’d prefer at 41ppm drop to 7ppm. I add liquid fert until I get a reading of 21ppm. The next day I test again and the nitrates are at 51ppm.

Is there something I’m overlooking or maybe my testing equipment is faulty?

My tank is setup as follows:
Size: 5 gallon tank
Stock: 1 betta, 5 chili rasbora, 1 mystery snail
Plants: Staurogyne repens, Anubias nana, dwarf vallisneria, Rotala indica bonsai, Rotala rotundifolia “orange juice”, echinodorus parviflorus, alternanthera renekii mini.
Light: Chihiros WRGB IISlim (setup so that ~70 par reaches the substrate)
Filter: Tru Nano Filter (Tru Aqua Design)
Media: filter sponges and Seachem Matrix
CO2: yes. About 1-2 bps
Maintenance: 2 water changes per week, vacuum gravel both times.

 

[zdrc]

What kind of tests are you using? How much substrate and decor do you have? Decor and substrate will reduce the functional water volume of a tank. For example, my 20 long probably holds more like 15 gallon of water due to the sand/rocks/equipment.

I have a 6 gallon shallow tank and it might only hold like 3 gallons of water.

 

[mannyaf7]

What kind of tests are you using? How much substrate and decor do you have? Decor and substrate will reduce the functional water volume of a tank. For example, my 20 long probably holds more like 15 gallon of water due to the sand/rocks/equipment.

I have a 6 gallon shallow tank and it might only hold like 3 gallons of water.
[/QUOT

What kind of tests are you using? How much substrate and decor do you have? Decor and substrate will reduce the functional water volume of a tank. For example, my 20 long probably holds more like 15 gallon of water due to the sand/rocks/equipment.

I have a 6 gallon shallow tank and it might only hold like 3 gallons of water.

I have a 1.5 inch layer of Carib sea eco complete for substrate. I’m currently measuring with a Hanna High range nitrate checker for nitrates and Low range phosphate checker for phosphate. Other parameters I measure with API freshwater master test kit. The nitrate checker says that it is for saltwater, but I followed another discussion that tested it for freshwater and found it was pretty accurate too:
Hanna HR Marine Nitrate Checker works on FW Too!
Also, it was between buying this checker, using the API one where the colors on the reference chart are all super similar, or buying a $300 Hanna nitrate photometer.
I’m including a pic of my tank to show decor, but even after calculating for that, I feel like my tank still has 3.5-4 gallons of water volume. Even if it had only 3.5 gallons, it still doesn’t make sense that it would go from 20ppm to 1.5ppm after a 2.5 gallon water change. Or from 41ppm to 7ppm after a 1 gal change. That’s a 93% and an 83% reduction in nitrate respectively.

 

[mannyaf7]

P.S. I did a 2gal water change today because nitrates were high and they went from around 50 down to 16ppm

 

[zdrc]

I have a 1.5 inch layer of Carib sea eco complete for substrate. I’m currently measuring with a Hanna High range nitrate checker for nitrates and Low range phosphate checker for phosphate. Other parameters I measure with API freshwater master test kit. The nitrate checker says that it is for saltwater, but I followed another discussion that tested it for freshwater and found it was pretty accurate too:
Hanna HR Marine Nitrate Checker works on FW Too!
Also, it was between buying this checker, using the API one where the colors on the reference chart are all super similar, or buying a $300 Hanna nitrate photometer.
I’m including a pic of my tank to show decor, but even after calculating for that, I feel like my tank still has 3.5-4 gallons of water volume. Even if it had only 3.5 gallons, it still doesn’t make sense that it would go from 20ppm to 1.5ppm after a 2.5 gallon water change. Or from 41ppm to 7ppm after a 1 gal change. That’s a 93% and an 83% reduction in nitrate respectively.

View attachment 1051426

It doesn't look like you have that much decor. I imagine you're pretty close to 4 gallons or so.

It could be because you're using the Hanna checker. It may not be that reliable for freshwater.

Honestly, most of the test kits available to hobbyists really aren't that accurate. I know Hanna has had some problems with reliability with some of their tests. On a reefing forum I'm on some people were complaining about inconsistencies in their magnesium test. Hanna is nice because it gives you a number with a decimal, but I would be somewhat suspicious of the exact accuracy of that number. It varies from test to test though, I believe they're the gold standard for low range phosphate tests. In some ways I like the API test because it's not lying to you about the accuracy of the test.

One of the weakest points for these tests is the tester themselves. Different people using identical tests on the same water sample can get different results. Slight differences in the angle of the dropper, the mixing of the testing liquids before administering the test, and the mixing of the liquids in the water sample can result in different results.

You could try the Salifert Freshwater Nitrate test. I have the marine test and I like it. It's another color based test though. The test is done such that higher resolution is obtained by looking through the side of the test vial vs. the top. I have pretty good color vision though, and I can totally understand wanting a digital read out instead of doing some color matching. Personally, I wish someone sold a titration test for nitrate. I really like the titration tests for KH.

 

[mannyaf7]

Ok, I’ll look into buying a Salifert test to compare the results and I’ll let you know.
I had an idea though. Could the gravel vacuum have anything to do with it? Hear me out: in the process of gravel vacuuming, the water that is drawn out of the substrate is replaced by some of the water from the water column. So, some of that high nitrate water gets temporarily “locked” away, leaving less nitrate in the water column for when I mix with fresh water. Then through the week it might just leech back out maybe, because I doubt the water wouldn’t eventually mix. Idk is my theory making any sense?

 

[zdrc]

Ok, I’ll look into buying a Salifert test to compare the results and I’ll let you know.
I had an idea though. Could the gravel vacuum have anything to do with it? Hear me out: in the process of gravel vacuuming, the water that is drawn out of the substrate is replaced by some of the water from the water column. So, some of that high nitrate water gets temporarily “locked” away, leaving less nitrate in the water column for when I mix with fresh water. Then through the week it might just leech back out maybe, because I doubt the water wouldn’t eventually mix. Idk is my theory making any sense?

I'm not sure if that makes sense. I don't really know how much water turnover there is in the substrate, but I think the volume of water in the substrate is too small to be a reservoir for high nitrate water. There might only be a few cups of water total in the substrate which would have almost no effect on the nitrate levels in the bulk water.

I think its much more likely that the Hanna tester is being unreliable given that you're using it on the wrong type of water. Just based on the numbers you've been giving me:

2.5 gal change 20 ppm -> 1.5 ppm
1 gallon change 40 ppm -> 7 ppm
dosing to 21 ppm -> 51 ppm after a single day
2 gal change 50 ppm -> 20 ppm

These numbers show a lack of consistency across the board, which would suggest that the test itself is at fault.

 

[mannyaf7]

Ok, I ordered the Salifert freshwater test kit and compare it.

 

[mannyaf7]

Heya, it’s a bit of an old convo, but after multiple tests, I managed to confirm that the Hanna nitrate test for salt water is about as accurate as the Salifert for freshwater. Results were always within about 2-3ppm of each other.