The bottom drain decides whether fish waste reaches your filters or settles on the pond floor. A drain that suits the pond keeps solids moving toward filtration, so the koi get cleaner water and you do less cleaning. One that doesn't leaves dead spots where debris piles up and has to be cleaned out by hand.
This guide compares two bottom drain lines Play It Koi carries: Aquadyne's Rhino drains and Dreampond's Koi Toilet drains. Both are conventional bottom drains that sit at the low point of the floor and feed filtration continuously, and both come with or without built-in aeration. The differences are construction, pipe sizes and price. KoiGuard comes up too, so there's a short note on where it stands. For a broader look at how bottom drains fit into pond design, see the complete bottom drain buyer's guide.
Why the bottom drain matters
A bottom drain sits at the deepest point of a koi pond and takes in the heaviest waste: fish solids, decaying organic matter and food that sinks before a skimmer can catch it. Without one, that waste breaks down on the floor, releasing ammonia and feeding algae. With one, it goes straight to a settlement chamber or filter, whether that's a rotary drum filter or a pressurized bead filter.
The drain usually costs less than the pumps and filters around it, but it decides whether solids reach filtration or rot on the pond floor. It's also the hardest part to change later, because a retrofit means draining the pond and opening up the floor, so it pays to get it right during construction.
Aquadyne Rhino drains
Aquadyne makes its Rhino drains in the US from Schedule 80 PVC. The parts are machine fabricated and every joint is welded by hand, with no injection-molded components. Play It Koi's own listing calls the Rhino II the most durable production bottom drain we've found.
Aquadyne Rhino II
The Rhino II is a 4-inch drain, sold with a built-in air diffuser or without one. It has thick flanges and a liner attachment ring that won't flex, distort or crack, and its wider machined flange gives you more margin for error when you cut the liner. It can be installed under or over a liner, and it works in gravity-fed and pump-fed systems.
It's a common pick for gravity-fed systems that run to external filters. A drain is the first thing set into a pond and the last thing you want to fail, so build quality counts.
Aquadyne Rhino II with air diffuser
The aerated version of the Rhino II adds an air diffuser to the drain. The airline plumbs straight into the drain's air bladder through a 1/2-inch input, so you don't have to snake tubing through the drain pipe, and a 1.5-inch check valve keeps water out of the airline. The column of bubbles rising above the drain pulls debris in from a wider area of the floor, including fine waste that would otherwise stay suspended, and it adds oxygen to the deepest water in the pond.
It needs an air pump. Drain air is usually set at about 25 to 40 liters per minute (LPM) per drain, roughly 0.9 to 1.4 CFM. Heavier fish loads and warmer water push the need up, while deeper water raises the pressure the pump has to push against. Our aerated bottom drain guide explains how drain aeration works and what to look for in an air pump.
Customer Randy Tan runs three Rhino II aerated drains in his gravity-fed pond, roughly 6,800 gallons and 5.5 feet deep. Read Randy's build story to see how he put it together.
Rhino I and Rhino Retro
The Rhino I uses the same welded Schedule 80 PVC construction and comes in 2-, 3- and 4-inch sizes, so a pond that calls for a 3-inch drain can still use an Aquadyne. For a pond that's already built, the Rhino Retro sits on top of the liner, held down by a ballast chamber you fill with stone or sand, and connects to the pump's suction line. It's also sold with a 9-inch air diffuser.
Aquadyne strengths
- Welded Schedule 80 PVC with no injection-molded parts, made in the US
- Thousands of installs, from home ponds to commercial fish-handling tanks
- A wide machined flange that leaves room for error when cutting the liner
- The air diffuser version puts the air right at the drain, so there's no separate air stone to place on the floor
- Sizes from 2 to 4 inches across the Rhino I and Rhino II, plus the over-the-liner Rhino Retro for existing ponds
Aquadyne: things to plan for
- Rhino drains cost more than Koi Toilet drains (current prices are in the comparison table below)
- The Rhino II comes in 4-inch only; for a 3-inch drain, use the Rhino I
- The aerated version needs its own air pump, which adds to the system cost
Koi Toilet drains
Koi Toilet drains are made by Dreampond. Despite the name, they're conventional bottom drains: water and waste flow into them continuously, the same as any other drain. They're injection molded, come in 3- and 4-inch sizes, and have a thick flange and liner ring for a good grip on the liner. Play It Koi's own pond runs a 3-inch Koi Toilet I aerated drain, with a PondMAX PA40 air pump feeding its diffuser.
Koi Toilet II
The Koi Toilet II is the non-aerated model, in 3-inch and 4-inch with dome versions. Its flared inlet is designed for smooth flow, it fits a standard Schedule 40 PVC elbow or coupling in its size, and it suits both liner and concrete ponds.
Koi Toilet I Aerated
The Koi Toilet I adds a built-in air diffuser, 9 inches across on the 4-inch model; the 3-inch model's diffuser has an internal check valve. A sweep-bottom design keeps debris from getting trapped, the screws sit in enclosed pockets, and a threaded center column makes installation and maintenance easier. The listing describes it as optimal for gravity-fed systems.
Koi Toilet strengths
- Both models come in 3-inch as well as 4-inch
- The aerated Koi Toilet I has the diffuser built in
- Costs less than a Rhino drain of the same size
- The Koi Toilet II suits both liner and concrete ponds
Koi Toilet: things to plan for
- The aerated model needs an air pump, like any aerated drain
- Sizes stop at 4 inches
Where KoiGuard fits
Play It Koi doesn't sell a KoiGuard bottom drain yet, so there isn't one to compare here. From KoiGuard, the store carries water treatments, beneficial bacteria and box-welded drop-in liners. If you're planning a build now, choose from the drains available today rather than waiting.
Side-by-side comparison
| Feature | Aquadyne Rhino II | Aquadyne Rhino II with air diffuser | Koi Toilet II | Koi Toilet I Aerated |
|---|---|---|---|---|
| Pipe size | 4" | 4" | 3" or 4" | 3" or 4" |
| Built-in air diffuser | No | Yes | No | Yes |
| Construction | Welded Schedule 80 PVC | Welded Schedule 80 PVC | Injection molded | Sweep-bottom body, enclosed screw pockets |
| Flow | Continuous | Continuous | Continuous | Continuous |
| Gravity-fed or pump-fed | Either | Either | Either | Either (listed as optimal for gravity-fed) |
| Price today | see current price | see current price | 3": see current price 4": see current price |
3": see current price 4": see current price |
Our pick for most new builds: Aquadyne Rhino II with air diffuser
For most koi pond builds, our pick is the Aquadyne Rhino II, and we'd take it with the air diffuser. It works in gravity-fed and pump-fed systems, the welded body is about as sturdy as production drains get, and the aerated version does three jobs in one fitting: better waste capture, better circulation and more dissolved oxygen at the bottom of the pond, where it tends to run out first. The full bottom drain collection has the other sizes and brands.
Our pick when a 3-inch drain suits the pond: Koi Toilet I Aerated
A 3-inch drain can keep up in a smaller pond with a modest fish load, and an aerated 3-inch drain will outperform a 4-inch drain running without air. The Koi Toilet I Aerated 3-inch is the drain on Play It Koi's own pond. A 3-inch gravity line carries about 1,800 GPH, half of what a 4-inch line carries, so size the pump to match.
How to choose
The bottom drain buyer's guide covers sizing and installation in more depth. These questions narrow down which drain fits:
- Will the drain be aerated? For most koi ponds it should be. If the budget forces a choice between a bigger drain and aeration, choose the aeration.
- Gravity-fed or pump-fed? Both brands work either way. In a gravity-fed system the drain pipe sets your flow ceiling: about 3,600 GPH for a 4-inch line and about 1,800 GPH for a 3-inch line, and less once biofilm narrows the pipe.
- What pipe size? Four inches suits most builds. A 3-inch drain can serve a smaller pond with a modest fish load, and the Rhino I and both Koi Toilet models come in 3-inch.
- How many drains? A common guideline is one drain per 2,500 to 3,000 gallons, so most ponds over 3,000 gallons need two or more, and long or irregular shapes may need extra. The bottom drain vs skimmer guide covers how to split flow between drains and skimmer.
- What filter is downstream? Both brands are continuous-flow drains, so they pair with a drum filter, a settlement chamber or a bead filter.
- Is the pond already built? A through-liner drain means draining the pond and cutting in. An over-the-liner drain like the Rhino Retro connects to the pump's suction line instead.
Frequently asked questions
Can a bottom drain be added to an existing koi pond?
Yes, two ways. A through-liner drain can be added by draining the pond and cutting into the liner or shell, which is a real project; the retrofit guide covers it. Or a retrofit drain such as the Aquadyne Rhino Retro sits on top of the liner, weighted with stone or sand, and connects to the pump's suction line. It's better than nothing, though not as good as a built-in drain, and it's far easier to put in a new pond than to add one later.
How many bottom drains does a koi pond need?
A common guideline is one bottom drain per 2,500 to 3,000 gallons of pond volume. Larger ponds or irregular shapes may need more to eliminate dead spots. Customer Randy Tan's pond, roughly 6,800 gallons, uses three drains to cover the whole floor.
Is the Rhino II with air diffuser worth the extra cost over the standard Rhino II?
For most koi ponds, yes. The diffuser improves waste capture, circulation and dissolved oxygen in one fitting, and it puts the air right at the drain without a separate air stone. If the budget forces a choice between a bigger drain and aeration, choose the aeration. Heavier fish loads and warmer water push the need for air up, while deeper water raises the pressure the air pump has to push against.
Does the Koi Toilet work with a pump-fed system?
Yes. The Koi Toilet is a standard bottom drain, so it can feed a pump-fed or a gravity-fed system like any other drain. The listing for the aerated Koi Toilet I describes it as optimal for gravity-fed systems.
What size pipe should connect to the bottom drain?
Match the pipe to the drain's outlet. Four inches suits most koi ponds, and it keeps a pump-fed system open to gravity-fed later. A 3-inch drain and line can provide enough turnover for a smaller pond with a modest fish load, but in a gravity-fed system a 3-inch line carries about 1,800 GPH against about 3,600 GPH for a 4-inch line. Keep the full size until you're above grade and away from the pond before reducing, so any blockage at the reducer is somewhere you can reach.
Does Play It Koi sell a KoiGuard bottom drain?
Not yet. Play It Koi carries KoiGuard's water treatments, beneficial bacteria and box-welded drop-in liners. If you're building now, plan your plumbing around the drains available today.