Hey,

How ya goin?

Good thanks!!!!

So, you’ve made it this far. You survived Parts 1 and 2. You’ve started testing your water chemistry properly, you're adjusting your alkalinity (KH), and you're paying attention to your dissolved minerals (GH).

And what's happened? 

The ammonia and nitrite have dropped?
The feed rates are climbing?
The fish are more active?
Their colours are popping?

The whole system just feels… healthier?

 You’re quietly high-fiving yourself in the fish room. and you should be 

(Just so you know, we’re high-fiving you, too.)

You’re probably thinking, "Sweet. My biofilter is dialed in now, right? Job done."

NOPE. NOT BY A LONG SHOT.

Welcome to the next level. You’ve laid the foundation, but now it’s time to squeeze every last drop of performance out of that engine you’ve built.

Let’s talk about the silent killer of biofilter efficiency: BOD.

The Real Enemy: Biological Oxygen Demand (BOD)

Biological Oxygen Demand is the amount of oxygen consumed by bacteria while they decompose organic waste. In simple terms, it’s the measure of all the crap in your water—uneaten food, fish waste, and decaying organic matter.

Your nitrifying bacteria (the good guys) need oxygen to convert ammonia and nitrite. But the heterotrophic bacteria (the clean-up crew that eats all the gunk) also need oxygen. A LOT of it.

When your BOD is high, it means there’s a massive population of these heterotrophic bacteria having an all-you-can-eat buffet. They multiply like crazy and start stealing all the available oxygen, suffocating the nitrifying bacteria you’ve worked so hard to cultivate.

High BOD means your biofilter is fighting for breath.

See the chart below.

On the left, you can see the impact BOD has on nitrification.

Not only nitrification increase as BOD decreases (increases grams of ammonia per m2 of surface area per day consumed), but inverseley as BOD increases, not only does nitrification decrease, but Oxygen requirements increases dramatically.

On the right, you can see how DO and ammonia concentration is intimately interlinked with removal rates of ammonia.

This is based on a 0.4ppm BOD level, very low, and would be considered a clearwater system. 

The Physical War on Waste

Before we even touch fancy equipment, let me be brutally clear. The single biggest source of out-of-control BOD is the sludge and biofilm hiding in your system. Especially in your pipes. That dark, slimy layer coating the inside of your plumbing is a non-stop BOD factory.

You DEFINITELY, without question, need to be physically cleaning your system.

• Scrub Your Tanks: At LEAST weekly. Get the algae, get the waste, get it out.
• Plunge Your Pipes: That’s right. Get a pipe brush or a plumbing snake and clean out your return lines, your drain lines, all of it. The amount of sludge you dislodge will horrify you, and that’s a good thing. Get it out of the water column and into your mechanical filter.

This isn’t a suggestion. It is the absolute foundation of high-performance water quality. You cannot out-tech a dirty system.

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STOP!!!!!

I don’t know if I made that clear enough.

The number of times I’ve heard
"What do I need to clean the mainline for?" or
"Nah, valves cost too much" or
"It’s too difficult to put in an access point"

Its staggering, no, its horrifying, bordering on offensive.

So people, just… don't.

Let me spell it out:

• If you have a system without direct access to the pipelines for cleaning them, you need to create one.
• If you are designing a new system, you NEED valves and/or access ports in the mainlines in order to be able to clean them.
• If you are having a system designed, and the designer does not include access ports for cleaning—be it valves (gate valves for access) or another means of access to plunge the pipework—WHICH AGAIN, IS NON-NEGOTIABLE, then you need to insist on it, or find another designer.

It’s a fact.
The pipe needs to be cleaned.
End of story.

Put in access points that allow you to do it. Please and thank you.

Moving on………….

 

The Heavy Artillery: Ozone (O₃)

Once you’ve committed to physical cleaning, it’s time to bring in the ultimate weapon for obliterating BOD: Ozone.

When it is applied properly and controlled with an ORP monitor, ozone is undoubtedly THE most powerful tool, hands down, you can add to a system to improve water quality. Forget the scaremongering. This is professional-grade water management.

Here’s what it does:

1. Microflocculation: Ozone causes tiny dissolved organic particles to clump together (flocculate) into larger particles. This makes them big enough to be captured by your mechanical filtration (drum filters, filter socks, etc.). It literally makes your filters more effective.
2. Biofilm Destruction: Ozone is a powerful oxidizer. It annihilates biofilm on contact, keeping your pipes, tanks, and equipment surfaces cleaner for longer. It kills the BOD factory at its source.
3. Crystal Clear Water: By removing dissolved organics, ozone dramatically increases water clarity and raises your Oxidation-Reduction Potential (ORP). A high ORP is a direct indicator of a clean, healthy, low-BOD environment.

The Supporting Arsenal: Your BOD-Busting Checklist

Ozone is the king, but it’s not the only tool. High-performance systems use a multi-pronged attack.

• Increase Flow & Aeration: Crank up the flow. Higher flow can result in higher process rates. In systems like MBBRs, this increases the abrasion between media pieces, sloughing off excess biofilm and keeping it at the optimal thickness. (Be careful though, you don't want to over-aerate your MBBR; it's a fine balance, so make small adjustments). In other designs, like trickle filters, increasing airflow through the packed bed can also significantly boost nitrification. There are lots of things you can do to manipulate biofilter performance, but each design has its own "tricks of the trade" that you can either take advantage of or manipulate to your benefit.
• Clean Your Drum Filter Screen: That screen is your front line against solid waste. If it’s clogged with biofilm and grease, it’s not doing its job. GET ON IT.
• Optimize ALL Mechanical Filtration: All other types of mechanical filtration need to be optimized, cleaned, or replaced more often. Filter socks, sponges, filter floss—these are not "set and forget" items. Waste sitting in your system is simply just going to dissolve and add to BOD. You need to collect it and get it out of the system entirely.
• Maintain Your Probes! Your pH, ORP, and DO probes are your eyes on the battlefield. If they’re dirty, they’re lying to you. Clean them weekly as per the manufacturer's instructions.
• Calculate Your CO₂ Levels: High CO₂ is another stressor that limits oxygen availability. You can calculate it if you know your pH and alkalinity. Need a hand? Give us a call with your alkalinity level in PPM and your pH, and we can walk you through it.

The Final Word:

Now, you must become a master of waste. Your job is to remove organic waste from your system faster than your system can produce it.

Attack it physically. Attack it chemically. Never let it build up.

That is the secret to unlocking the true, full potential of your biofilter.

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Next Episode:

Biofilter types, the difference, the advantages, the disadvantages, the ways to manipulate their performance

We haven't yet gone into how BOD impacts different biofilter types—from moving beds to static media and trickling towers. Each one has a unique relationship with biofilm thickness and oxygen demand. 

#BTA #aquaculture #biotechaquatica #BOD #RAS #aquarium #waterquality