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Why are plants dying in only this tank? Even duckweed!!

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I'd like some input on something. 

Some background info:  
I have had this 10 gallon tank setup for about 9 months. It houses blue dream shrimp and cory fry up to roughly 2 weeks old. It was originally barebottom but I added black sand about 3-4 months ago. The water source is aged tap water.  I let the water sit for 24 hours and then scoop out the water on the top only. This is to lower the gh for the shrimp.  All animals are doing great and thriving!

The problem is plants. I have had flame moss, some Java moss, and marimo moss balls in there since the beginning.  At first all were doing fine but the past few months I have noticed that the amount of moss is shrinking. With one exception, all other plants I have introduced to the tank have the following effect:  After the plant is introduced, I get a spike of brown algae on the walls. Shortly after, the plant dies away. I put a nerite snail in the tank to eat the brown algae. After the plant fully dies, the brown algae does not come back. 

The one plant that has had a different reaction is hygrophila polysperma. I have had some in the tank now for about 1 month. It has not grown as quickly as it does in my other tanks but it does grow. Something interesting to note though is that ever since introducing it to the tank, I do still get small patches of brown algae but not as much.  

I would really like to add more plants to this tank so I would like to figure out the issue. I have tried so many easy plants and they all die. Even duckweed died!  I'm wondering if it's something about the process I'm doing for the source water or if there is something else unique to this tank as I don't have this issue in any other tank. My 29 gallon has lots of very thriving plants. 

While testing the paramaters I found something of interest that may point to the issue. The tank is showing a PH of 8.2 yet a KH of 7. The source water test PH of 8.2 and KH of 14. Could something, maybe the shrimp, be depleting the KH and therefore making it hard for the plants?  

I'm told that it's actually not all that uncommon of an issue in the saltwater world to have a high PH yet low KH. It makes me wonder how to go about a solution. How can you raise the KH without raising the PH?  I'm considering trying a very small amount of crushed coral and seeing how that works out. I would test both KH and PH daily to see the progress. I can always add more crushed coral if it seems to need it. I'm using sponge filters so I'm thinking of just putting the crushed coral in a media bag and setting it on top of the substrate. A last ditch solution may be using RO water but I hope to not have to do that. 

So...what do you think?

What might be my issue with the plants?  Is it the PH/KH difference?

How would you go about raising KH without raising PH?

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On 2/12/2023 at 7:06 PM, Mmiller2001 said:

Why would you want to raise KH? Plants like lower carbonates and softer water.

The issue is mainly that the kh and ph do not line up. Tbh, I'm not clear on what the issue with that is but I'm told by a few people it can cause issues and is a common issue in the saltwater world. 


On 2/12/2023 at 7:11 PM, Theplatymaster said:

@Cinnebunswhat  are your lighting hours? ive had brown algae bursts because someone left the light on, on a tank  overnight.

It's just a white hood light, not full spectrum, and it's on roughly 8 hours a day. 


On 2/12/2023 at 7:33 PM, YYC Carrie said:

I think the shrimp use the KH for their shells. Them and snails. someone with more experience please correct me if I'm wront.

That what I was wondering too. Snails actually use the calcium in GH as do shrimp for their exoskeleton but idk if they also use KH. 


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On 2/12/2023 at 11:02 PM, Preston John said:

@Cinnebuns let’s see some pictures of the tank in question. That will help us help you;)

Here's a video. Its fun to also see all the cute baby panda cories!


Here's a pic of the whole tank



And specifically of the few brown algae spots there currently.  Usually the brown algae is worse immediately after adding plants and then dies away itself. This time a small amount of it has been around for awhile. It may be because of the hygrophila in there maybe?



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On 2/13/2023 at 11:28 AM, knee said:

This is the first time I have heard that aging water lowers gh. I always thought gh remained the same unless filtered with RODI.

What is your GH?

Have you tested for phosphates? Have you tested other tanks and compared results?

Aging the water isn't what lowers the gh. What happens is the minerals sink to the bottom. You then scoop water off the top only and it's lower gh. 

My gh from the tap is 21. The gh in the tank, and from the source water is 14. 

I haven't tested phosphates but I've considered it. I also have a phosphate pad. 

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