Plankton Tank

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Some observations from my experimental plankton tank.

Tank Setup

I have a 55g display + 15g system, with a vortex impeller circulation pump (600GPH w/two eductors) and a 100GPH sump pump with a vane impeller.

I have only 5lbs of live rock in the entire system, since I am studying a low-predation plankton system.

The substrate is egg crate on the glass, with a thin sand bed (1") of 50% crushed coral and 50% argonite sugar sand. The egg crate forms pockets of anerobic areas in the thin substrate.

The sump first-stage (heater and skimmer) has a 2" sand bed for additional nitrogen fixing.

Parameters (constant throughout the experiment):

S.G.: 1.024
pH: 8.4-8.3
Temp: 72F-82F
Amm/Nitate/Nitrite = 0 (occasional Ammonias spike when changing parameters, but that's it).

The refugium has a small amount of chaeto, but it grows very slowly due to the high quantity of phyto plankton which get first dibs on the nitrogen products.

All water is Pittsburgh City tap (tested to have < 0.1ppm Phosphorus, and no detectable Copper) with 1tsp/5gallons of "Start Right" dechlorinator.

My topoff rate is approx 0.75 gallons/day, varying by ambient humidity.


  • If you don't have very much live rock to act as a phytoplankton predatory system, your tank can stay pea-soup green for weeks, killing all green macro algae due to light starvation.
  • Red macro-algae is unaffected by a phytoplankton bloom - in fact, it seems to grow pretty well under those conditions. Its chlorophyll appears to be able to utilize the green wavelengths that the phytoplankton does not absorb.
  • Pea-soup green phytoplankton will absorb almost all blue light within 12 inches of water column.
  • A Seaclone 150 skimmer makes almost no impact on plankton population.
  • A impeller-based sump pump kills almost everything over 2mm in size, but does not impact anything under 1mm.
  • Actinic-only lighting will reduce a phytoplankton bloom's opacity by 50% every week, but the opacity will rebound by 100% every day that full spectrum is used, until it reaches maximal opacity. Opacity does not equal population density - they just fade, they don't die.
  • The fastest way to crash a phytoplankton system, and switch it over to a zooplankton system, is to add 1Tbsp/20 Gallons of dissolved table sugar.
  • Full conversion from phyto-plankton dominated (green water) to zooplankton dominated (yellow water) took under 36 hours.
  • A sugar-invoked zooplankton system will eat all of your calcerous red algae within four to five days. It's actually pretty amazing. Scary, but amazing.
  • Chaeto does not grow at all, and in fact slowly dies, in a phytoplankton system.
  • Chaeto appears to double in size every 72 hours in my zooplankton converted system.

Current Status

Currently converting system to a clear-water Marine Tank.

Proposed changes:

  • Additional main tank livestock (1 brittle star, 1 porcelain crab, 1 sea cucumber)
    • Reason: generate additional nitrogen for macro-algae growth.
  • Additional refugium livestock (3 handfuls Chaeto, 2 Peppermint Shrimp)
    • Reason: provide additional competition for nutrients (shrimp clean the chaeto)
  • Add 50lbs of Tufa calercous (dead) rock
    • Reason: Determine 'infection' rate of Tufa with the the current live-rock ecosystem
  • 20 Gallon water change every week for four weeks
    • Reason: Reduce phytoplankton population via physical removal
  • Increase feeding (5g of scallops every 3 days)
    • Reason: Feed the brittle star.