What goes inside the tank.
The two things a Designer build can’t guess for you: which filter media to stack, and which rocks & wood are safe for your water. Browse them here, then take it into a build.
Filter media, in order
Water must meet the media in one direction: mechanical → biological → chemical. Sieve the solids, then feed the bacteria, then polish — chemical last and only when there’s a job for it. Solids first protect everything downstream.
Strip the solids up front
Coarse before fine catches poop, uneaten food and detritus so it can’t rot in the tank or blind the biomedia downstream. A clogged sponge becomes a nitrate factory — clean it on schedule.
Feed the nitrifying bacteria
Now-clear water passes over high-surface-area biomedia where bacteria turn ammonia → nitrite → nitrate. Putting bio after mechanical keeps the colony from silting up.
Polish what’s left — as needed
Carbon / Purigen / GFO finish the job (tannins, phosphate, meds). It’s a finisher, not a permanent stage, and goes last so the pricey adsorbent isn’t wasted on gunk the sponge should have caught.
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Bacteria live here — big porous surface means more bacteria means more waste processed. Rinse in old tank water, never tap (chlorine/chloramine kills the colony you spent 4–6 weeks growing), and never replace it all at once — swap a portion at a time so you never lose the whole cycle.
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Layering, per filter type: a canister packs coarse → fine sponge → ceramic/porous bio (biggest basket) → floss pad → chemical last. A sump runs socks → skimmer/refugium → bio → a reactor on the return for GFO/carbon. An HOB combines sponge + a dedicated bio section (keep it when you swap the cartridge — many stock cartridges are carbon+floss, so binning them monthly also bins your bacteria). A sponge filter is biological only. Confirm ammonia/nitrite = 0 with a test kit before stocking.
Hardscape — wood & stone
Wood leaches tannins (cosmetic, not harmful — good for blackwater) and mostly needs a pre-soak. Stone is decided by one question: does it buffer your water, or is it inert? Test every mystery rock before it goes in.
Drip a little household vinegar on a hidden spot of any rock and watch:
Carbonate → buffering
It dissolves slowly and raises KH / GH + pH over time. Fine for hard-water tanks; wrong for shrimp, discus, blackwater and high-tech soft-water plants.
Inert → safe anywhere
Chemistry-neutral. Doesn’t move your water. Dragon stone, lava rock, slate, quartz and basalt sit here — the safe default for almost any tank.
Do this to every unknown rock — especially anything you collected in the wild (scrub it first, no soap, ever).
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Inert · safe anywhere dragon/ohko stone, lava rock, slate, quartz, basalt, most river pebbles, ceramic/porcelain. These don’t move chemistry.
Buffering · hard-water only seiryu, limestone, Texas holey rock, marble, crushed coral, aragonite. Great for Rift cichlids & brackish; keep out of shrimp / discus / blackwater / high-tech soft-planted tanks.
Never in a tank metal-bearing / ore rocks (copper is lethal to inverts), painted / dyed / resin-coated “craft” rocks, seashells or coral in a soft-water tank, and any unknown wild rock you haven’t scrubbed + fizz-tested. Skip anything with metallic veins, a sulphur smell, or that crumbles.