By 苏剑林 | December 07, 2020
Recently, I’ve fallen into the rabbit hole of fishkeeping and set up a 60cm × 40cm ultra-white tank, primarily for co-habitating turtles and fish. I personally pursue a natural and biomimetic style, so I hope to establish a relatively stable ecological environment within the tank. Of course, those are actually just excuses; the real reason is that I’m too lazy to change the water and don’t want to wash filter sponges. Therefore, I thought about relying on the ecosystem's own self-purification capabilities to extend intervals between water changes. To this end, I referenced online materials to set up a Same-Path Bottom Filter and made some modifications based on my own experience.
Same-Path Bottom Filter
Simply put, a Same-Path Bottom Filter consists of pipes buried under the substrate. Its characteristic is having one outlet and multiple inlets, where the distance from each inlet to the outlet is equal ("same-path"), and these inlets are distributed as evenly as possible. By burying such pipes under the substrate and using a water pump to draw water from the outlet, the same-path design ensures that the suction force at every inlet is identical. This achieves uniform water suction from the substrate, effectively "activating" the substrate. This is the concept behind a (positive suction) Same-Path Bottom Filter. The implementation of this design is the "H-shaped" (工-shaped) same-path piping, as shown in the figure below (doesn't it give off a sense of fractal déjà vu?):
Same-path pipe schematic
There are already many tutorials online for building Same-Path Bottom Filters. My main difference is that I replaced all the substrate with 1cm-sized ceramsite (totaling 40 jin/20kg). Ceramsite can be used as both substrate and filter media; it has a porous structure that can be used for water purification and bacteria cultivation. In contrast, common substrate used in standard bottom filters, like iron gall sand, does not serve this purpose. Additionally, I chose ceramsite because it is uniform and appropriately sized, so it can be used to fill the gaps between the pipes. This makes the exterior look like just a thick layer of ceramsite substrate, giving it a "hidden mechanism" feel that is clean and aesthetic.
Same-path filter - pipes and bed
Same-path filter - partition and ceramsite
Same-path filter - pump and basking platform
Same-path filter - layout and aquatic plants
Tank Landscape
It is not hard to understand that the Same-Path Bottom Filter itself has no filtering effect; it only facilitates the attachment of particles in the water to the substrate, which is the only function I need from it. The water coming out of the pump flows directly back into the tank without passing through any other filtration, so there is no need to wash filter sponges. Since the bacterial rings and ceramsite both have adsorption properties, keeping the water clear is not an issue (as long as the substrate is not disturbed). Furthermore, both have bacteria-cultivating properties that can foster enough beneficial bacteria (such as nitrifying bacteria) to complete the decomposition of harmful substances like ammonia and nitrite. At the same time, there are aquatic plants in the tank, which also help absorb ammonia and nitrates. As a result, the water change cycle can be greatly extended—basically, just changing a bit of water every month or two is enough.
Currently, the tank houses 5 baby Reeve's turtles, 8 Chinese Paradise fish, and 4 small golden loaches. Later, I plan to add some Corbicula clams and cherry shrimp. The plants include Elodea, Hornwort, and Sweet Flag (Acorus calamus). The Sweet Flag is anchored using a plastic bottle, while the others are basically just tossed into the water, primarily for water purification rather than aesthetics for now. One of the small turtles has developed skin rot on its foot, so I DIYed an isolation box to keep it separate; otherwise, the Paradise fish keep picking on it. This is the current status.
Ecological tank - top view
Ecological tank - front view
Ecological tank - side view
Ecological tank - isolation box