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Why are filters recommended based on tank volume?


Millie.

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Right, I'm getting ready to feel silly because I've missed something obvious :doh11: but I don't understand the system for recommending filtration. I know for goldfish it is around 10x the tank volume...but why is it based on the amount of water at all?

Two goldfish in a 40 gallon produce the same amount of ammonia as two fish in an 80 gallon...so...why don't we recommend filtration based on the number of fish? I know filters are sold for being suitable for 20g, 60g, 120g tanks etc but wouldn't a system where filters are  recommended based on a tanks bio load make more sense? I just bought two filters for my tank online and realised that the information that seemed important was l/ph...but surely this is secondary to the amount of media the filter holds? :hummm I mean, wouldn't a filter with a small media tray that was able to 'suck' water in and out quickly still perform worse than one which could hold lots of media but wasn't so fast on the l/ph front? I don't have a system to propose as an alternative but this system doesn't make much sense to me either :idont

 

???confused???

 

 

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That's a good question.  Don't feel silly.

 

And I'm not sure I have the answer but this is my best guess.  :rofl

 

You said it yourself--it's a system for RECOMMENDED filtration.  It's merely a guideline.  And the 10 times filtration assumes "normal" stocking and HOB filtration.  With canister filters, we recommend 5-7 times filtration.  For heavily stocked tanks, we'd recommend more.

 

The other thing you need to consider is the size of the fish.  I have 6 fish ranging from 20 to 135 g.  Obviously a tank full of 20 g fish is going to produce much less waste than the same volume tank with the same number of 135 g fish. 

 

They are guidelines--filtration guidelines, stocking guidelines, etc.  :D  It's not black and white.

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The reason I come up with is pretty much the same as said above.  It is a general guideline, provided to the general public.  The general public is not a group of people that I consider to be real sharp, so the label on the box has to be as simple as possible.

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Well, you want the tank to be filtered quickly and completely.

 

With an HOB, you don't have very much room for media. Like you said, media's important in filtering. With a HOB, that's why we go for 10x the tank's volume per hour. We want everything quickly and efficiently cleaned.

 

With a canister, there is more room for media, so we can lower this to 5-7x per hour and still get a clean tank. 

 

These are recommended based on bioload as well, and since goldfish have a higher bioload than other fish, they are pretty extreme when compared to a tropical tank, right? :)

Edited by ChelseaM
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Two goldfish in a 40 gallon produce the same amount of ammonia as two fish in an 80 gallon...so...why don't we recommend filtration based on the number of fish? 

 

Let me try to present it this way, to see if it makes sense:

 

1. Firstly, you are right in that it's the mass of the fish (and how much you feed) that determines the waste output, and hence the amount of filtration that is needed.

2. We actually make filtration/filter recommendations based on the (max) number of adult fish that should be in a tank. So, in a 40 gallon tank, the max that you should really have in that tank is 2 adult goldfish, and for that sort of estimated fish mass, filtration from HOBs totaling up to 400 gph should be able to handle the bioload with ease. By the same reasoning, you will want 800 gph at least for the 80 gallon tank, and that level of filtration should be sufficient for 4 adult fish. Can you have less filtration in the 80 if you only have 2 fish or smaller fish? Probably, but at least for me, when I design something, I design it for max potential. 

3. The gph is just a stand in for how big the HOB will be, and consequently how much media will be there.

 

So, it's by far not a perfect measure, but over the course of years, we know that this number works, and works well, in the sense that once the cycle is established, it is quite stable. Will less filtration work? Possibly. Do you need more? No. Can you have more? Certainly. On my tanks, I have a combo of canisters and HOBs that add up to about 15-17x gph. :)

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Thanks everyone, really good answers and I do kind of get it...but as you all pointed out, there's so many variables to the rule, even though it is just a recommendation. How big are the fish, how many, what type, how much fed... couldn't the filter boxes just say, 'Fluval C4, for 28 total inches of fish in tank' etc  :lol:  

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Thanks everyone, really good answers and I do kind of get it...but as you all pointed out, there's so many variables to the rule, even though it is just a recommendation. How big are the fish, how many, what type, how much fed... couldn't the filter boxes just say, 'Fluval C4, for 28 total inches of fish in tank' etc  :lol:  

Sorry, my first post was based on reading your post wrong.

And ya can't go by inches either! Consider a skinny singletail goldfish vs a fat oranda. The same length oranda would weigh probably 3 times as much or more. Maybe the best measurement would be by the weight of the fish?

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Its definitely a guideline for the max number of adult fish you are supposed to have.  When I had 3 adult fish in my 40b (instead of 2)  I had to bump it to 15x to keep the tank clean ;)

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Thanks everyone, really good answers and I do kind of get it...but as you all pointed out, there's so many variables to the rule, even though it is just a recommendation. How big are the fish, how many, what type, how much fed... couldn't the filter boxes just say, 'Fluval C4, for 28 total inches of fish in tank' etc :lol:

Sorry, my first post was based on reading your post wrong.

And ya can't go by inches either! Consider a skinny singletail goldfish vs a fat oranda. The same length oranda would weigh probably 3 times as much or more. Maybe the best measurement would be by the weight of the fish?

Also filters are designed for fish in General not just goldfish. A tropical rank stocked with small tropical fish does not have the same bioload ad one srocked with goldfish or other larger fish. Tink how many neon tetras would it take to equal the mass of a single adult goldfish? 50 maybe? They are around 1.5" long so thats 75" of fish! Companies do the best they can to make a general guide to how many gallons a filter iss good for. For some fish it may be over kill for other like goldfish we must make adjustments to the recommendations to accommodate our choice of fish.
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