Help Water change day: how to fert after?

[nivliw]

Is there a magic number of fert to give after a what change? Is it 3x the amount that I give during any day of the week?
Example: I give 1ml every day then after a 50% water change should I give 3x (so 3ml?)

 

[CincyScaper]

This is a fav. Topic of this forum and you are going to get a lot of detailed responses.

I am lazy and going to direct you to - Why nutrient level stability matters for planted aquariums

I use a zero bound system personally.

 

[nivliw]

This is a fav. Topic of this forum and you are going to get a lot of detailed responses.

I am lazy and going to direct you to - Why nutrient level stability matters for planted aquariums

I use a zero bound system personally.

Thanks you, I did already see the 2hr aquarist website, but what I was wondering is if the triple dose is the standard to keep the balance after a 50% water change or if it’s not always like that.
While I’m at it I also wanted to ask how do I know if I do exactly a 50% water change, my rio 180 supposedly has 180 liters however when I change 80 liters which should theoretically be less than 50% I see the level of the water dropping way more than half

 

[JayP]

Thanks you, I did already see the 2hr aquarist website, but what I was wondering is if the triple dose is the standard to keep the balance after a 50% water change or if it’s not always like that.
While I’m at it I also wanted to ask how do I know if I do exactly a 50% water change, my rio 180 supposedly has 180 liters however when I change 80 liters which should theoretically be less than 50% I see the level of the water dropping way more than half

Determining ACTUAL tank volume with a water change and a TDS meter

 

[JayP]

Is there a magic number of fert to give after a what change? Is it 3x the amount that I give during any day of the week?
Example: I give 1ml every day then after a 50% water change should I give 3x (so 3ml?)

Are you actually dosing 1ml per day? What fert are you using? 1ml per day for a tank that size is rather low.

 

[nivliw]

Are you actually dosing 1ml per day? What fert are you using? 1ml per day for a tank that size is rather low.

Ok so… I was dosing 3ml/day of apt E however I was getting staghorn algae mostly on reneickii and Montecarlo my nitrates were 30+ so after that I dosed 1ml a day and with water changes I’m now at 6ppm of nitrates and 1.5 phosphate, so I guess the unbalance between them was present even before , before I only tested nitrates but yeah

 

[nivliw]

 

[Koan]

@Naturescapes_Rocco has put together a calculator, specifically to help you figure out how best to manage this!

Resource Thread 'Rocco's Nutrient Accumulation Calculator'

Oct 17, 2025

Running a tank with BDBS (inert sand substrate), no root tabs, no aquasoil, and only dosing DIY ferts via EI/"non-limiting" methods has been extremely enlightening (and successful!). To help my journey, I created a tool to help myself and others who use weekly water changes and manual fert dosing:

Rocco's Nutrient Accumulation Calculator​

Journal for this tank is here!​

​

Who is this calculator for?​

For anyone...

• Naturescapes_Rocco

• 
• 

Paging Rocco, @Naturescapes_Rocco to the white courtesy telephone please

 

[CincyScaper]

Thanks you, I did already see the 2hr aquarist website, but what I was wondering is if the triple dose is the standard to keep the balance after a 50% water change or if it’s not always like that.
While I’m at it I also wanted to ask how do I know if I do exactly a 50% water change, my rio 180 supposedly has 180 liters however when I change 80 liters which should theoretically be less than 50% I see the level of the water dropping way more than half

One does not have to be so specific in my opinion. Stability is more important than the actual volume. So if you are doing 80 liters consistently that should be ok.

I would stick to one specific dosing and stick to it, so that it becomes a constant rather than a variable parameter. Your algae could have multiple other causes such as too much light, Co2 imbalance etc.

The fact that you have gone from 30+ nitrate to < 10 nitrate is going to cause adaptive stress and I would expect this untill new growth takes over and might need lot of trimming.

Since this is likely going to be a reset for you, here are some options to get your nutrient level stable,

First figure out, how much nitrate is absorbed by your system in a week, and then decide on how much nitrate level you are going to target. I have seen people target 10 - 20.

@Naturescapes_Rocco has a detailed calculator for nutrient accumalation - Resource - Rocco's Nutrient Accumulation Calculator

My personal preference when using APT 3, is to target < 2 of nitrate which essentially means the system absorbs about for example 1 ppm of nitrate/day and i provide 1 ppm of nitrate/day; so essentially i don't have to triple dose or double dose after a water change. This will give less robust growth than targeting higher nitrate, but less trimming for me. I have lights set lower to accomadate this. This is not a perfect system, but there is no perfect system.

Hope this helps. It is a great hobby and there are people here with lot of experience and commitment to this hobby like @JayP

 

[nivliw]

One does not have to be so specific in my opinion. Stability is more important than the actual volume. So if you are doing 80 liters consistently that should be ok.

I would stick to one specific dosing and stick to it, so that it becomes a constant rather than a variable parameter. Your algae could have multiple other causes such as too much light, Co2 imbalance etc.

The fact that you have gone from 30+ nitrate to < 10 nitrate is going to cause adaptive stress and I would expect this untill new growth takes over and might need lot of trimming.

Since this is likely going to be a reset for you, here are some options to get your nutrient level stable,

First figure out, how much nitrate is absorbed by your system in a week, and then decide on how much nitrate level you are going to target. I have seen people target 10 - 20.

@Naturescapes_Rocco has a detailed calculator for nutrient accumalation - Resource - Rocco's Nutrient Accumulation Calculator

My personal preference when using APT 3, is to target < 2 of nitrate which essentially means the system absorbs about for example 1 ppm of nitrate/day and i provide 1 ppm of nitrate/day; so essentially i don't have to triple dose or double dose after a water change. This will give less robust growth than targeting higher nitrate, but less trimming for me. I have lights set lower to accomadate this. This is not a perfect system, but there is no perfect system.

Hope this helps. It is a great hobby and there are people here with lot of experience and commitment to this hobby like @JayP

As for co2 imbalance, what is it exactly?
Like If I have 40 ppm when the light turns on and 50 before it turns off is it considered imbalance?

 

[Unexpected]

As of late, I'm 2.5x dose after a water change on Macros. 40 to 50 is solid. Ideally, have 50 all day.

 

[CincyScaper]

As for co2 imbalance, what is it exactly?
Like If I have 40 ppm when the light turns on and 50 before it turns off is it considered imbalance?

That seems better stability than mine.

 

[Naturescapes_Rocco]

One very easy way to fertilize is through large weekly water changes (50-70%), and dose the incoming water. This system works REALLY well for low tech tanks;

In my bowl tank, I remove 50% of the water, and dose 1mL of APT 3 for every 1 gal of water I'm replacing. I also add 30ppm Ca via CaSO4 and ~4ppm Mg via MgSO4 (because APT already has magensium).

So I dose the incoming water volume with my fertilizer. Works really well for low energy systems.

In high energy systems I do something similar; large water changes with an initial large front load of fertilizer. However, I also provide daily dosing of macros and micros via autodosing pumps to help keep levels stable.

In low tech systems, plants don't usually consume enough nutrients to cause major fluctuations, so every week you are basically "topping off" the water column after your water change.

In high tech systems, plants can really quickly consume all the available nutrients, so I like to provide some daily dosing to keep nutrients available, even after the initial front load following the water change.

 

[nivliw]

One very easy way to fertilize is through large weekly water changes (50-70%), and dose the incoming water. This system works REALLY well for low tech tanks;

In my bowl tank, I remove 50% of the water, and dose 1mL of APT 3 for every 1 gal of water I'm replacing. I also add 30ppm Ca via CaSO4 and ~4ppm Mg via MgSO4 (because APT already has magensium).

So I dose the incoming water volume with my fertilizer. Works really well for low energy systems.

In high energy systems I do something similar; large water changes with an initial large front load of fertilizer. However, I also provide daily dosing of macros and micros via autodosing pumps to help keep levels stable.

In low tech systems, plants don't usually consume enough nutrients to cause major fluctuations, so every week you are basically "topping off" the water column after your water change.

In high tech systems, plants can really quickly consume all the available nutrients, so I like to provide some daily dosing to keep nutrients available, even after the initial front load following the water change.

How

One very easy way to fertilize is through large weekly water changes (50-70%), and dose the incoming water. This system works REALLY well for low tech tanks;

In my bowl tank, I remove 50% of the water, and dose 1mL of APT 3 for every 1 gal of water I'm replacing. I also add 30ppm Ca via CaSO4 and ~4ppm Mg via MgSO4 (because APT already has magensium).

So I dose the incoming water volume with my fertilizer. Works really well for low energy systems.

In high energy systems I do something similar; large water changes with an initial large front load of fertilizer. However, I also provide daily dosing of macros and micros via autodosing pumps to help keep levels stable.

In low tech systems, plants don't usually consume enough nutrients to cause major fluctuations, so every week you are basically "topping off" the water column after your water change.

In high tech systems, plants can really quickly consume all the available nutrients, so I like to provide some daily dosing to keep nutrients available, even after the initial front load following the water change.

how much do you front load? Is it 3 times the normal daily dose?
Also about remineralizijf I usually bring the water back to 120 tds after I add it to the tank by using apt sky, I saw mj aquascaping the YouTuber suggesting this value however I’m not sure if that’s the right way.
I have measured the gh and I sit at 7 gh