WATER CHANGE QUEST

[Akshay]

This is my goldfish tank. I usually do a 50–60% water change once a month, but not because NO3, PO4, etc. become an issue.

Even after 4 weeks, NO3 is usually around 5–6 ppm and PO4 around 0.2–0.4 ppm.

The main reason I settled on doing water changes once a month was that, at around the 4-week mark, I would usually start seeing some brown diatom algae and green dust algae.

I dose ferts daily in this tank—macros in the morning and micros in the evening.

My weekly dosing is roughly:

• NO3: 8 ppm
• PO4: 2.8 ppm
• K: 40 ppm
• Fe: 0.17 ppm

Other water parameters are:

• Ca: 30 ppm
• Mg: 10 ppm
• pH: around 7.6–7.7
• KH: 3 dKH

This time, though, I couldn’t change the water at the 4-week mark, so I just let the tank run. It has now been 6 weeks without a water change, & I’ve had no algae whatsoever.

Now, I understand that water changes aren't just about NO3 and PO4. They also help dilute/remove DOCs, hormones and other things that may accumulate over time. but i think DOCs cause diatom algae hence the appearance of diatom would mean ( for me) water change

So I basically have two questions:

1. Is there any practical way to measure DOCs in aquarium water?
If there is, I’m thinking that might give me a better idea of when the tank actually needs a water change rather than just following a fixed schedule.

2. Could I realistically stretch my water changes to once every 2 months or so?
It’s only 4 goldfish, around 3–4 inches each, in roughly 300 L of water. Nutrients don’t seem to be building up, and the tank seems perfectly fine even after 6 weeks.

What do you guys reckon? Would you be comfortable running it for 2 months between water changes?

 

[Art 174]

Hi, in your case, I would just a good activated carbon and replace that every 2-3 weeks. Small amount.

That should remove tannins and DOC from the water stretching the need for more frequent water changes. However, while this should allow you to stretch the time between water changes, I would not stop them all together.

 

[Akshay]

Hi, in your case, I would just a good activated carbon and replace that every 2-3 weeks. Small amount.

That should remove tannins and DOC from the water stretching the need for more frequent water changes. However, while this should allow you to stretch the time between water changes, I would not stop them all together.

Dosen't Carbon remove Chelates too?
Have thot of purigen for the same purpose

 

[Art 174]

The wrong type of activated carbon, in my experience, will remove chelated iron relatively quickly. The reason is that micropores will capture the large chelate molecules.

The right type of activated carbon is macropore carbon such as that made from wood. AquaChar is a good example. Due to the larger pore size, it is less likely to capture the chelates. Its impact is minimal, contrary to the common belief, especially the way we dose our tanks. See here for more: Activated Carbon, purigen, are these useful for planted tanks?

Purigen is a synthetic polymer that removes nitrogenous organics. It removes a smaller subset of what’s in the water. Even Seachem says that Purigen and activated carbon are compatible to remove a broader amount of unwanted dissolved substances from the water. I have found good quality carbon does a better job of removing tannins than Purigen.

I used to use both with carbon being changed every week or two and Purigen lasting longer. This gave me crystal clear water maximizing PAR at the bottom and gave me a good buffer if I missed a weekly water change.

With your setup, this would allow you to extend water changes without material impact to your nutrients.

If you think about water changes, a 50% water change would remove 50% of the DOC in the water. If you were running activated carbon and/or Purigen, the 50% of the remaining DOC would be brought down much lower without impacting the chemistry of the water and with minimal, if any, impact to nutrient levels.

ADA has followed this type of method for years and years. They’ve had pretty good success.

 

[CincyScaper 151]

You could check your TDS after a regular water change and see how it creeps up weekly and at what TDS you get the diatoms. TDS is non-specific but can give you a good trend if you have stable water quality and all measurable variables such as nitrates are stable.