This is the 5-gallon opae’ula tank on my desktop, right next to the south-facing window. This photo was shot at around 6:30 this morning. I replaced the sponge filter with an undergravel filter (UGF) about a week ago.
The conversion required dismantling and removing the coral condo, moving the shrimp to a holding tank, pouring the water into plastic bottles, removing the sponge filter, inserting the UGF system, adding gravel, rebuilding the condo, pouring the water from the bottles back into the tank, and moving the shrimp back in.
The impact was immediate. The opae seemed livelier from the get-go. They’re behaving more like the shrimp in the first 10-gallon, actively swimming about and foraging. This conversion means that I no longer use sponge filters in any of my opae’ula tanks.
My guess is that sponge filters systematically remove critical bacteria, gradually turning the tank into a virtual dead zone with little or no activity. UGFs, on the other hand, seem to provide biological filtration without removing good bacteria, creating a habitat that simulates natural conditions in anchialine pools.
I replaced the AquaClear 5 with an AquaClear 10 air pump. Compared to sponge filters, UGFs require a much more powerful pump. From what I’m seeing, a rough rule of thumb would be to double the capacity of the pump for the designated tank size. Thus, use a 10-gallon pump for a 5-gallon tank, a 20-gallon pump for a 10-gallon tank, etc. When using a pump that exceeds this 2-to-1 ratio, you can always reduce the powerful outflow by using the cartridge filter.
The UGF is Lee’s model 13205. The design is slightly different from the Petco UGFs in both 10-gallons. The lift tubes are the same diameter so the outlets at the top from both models are interchangeable. The Lee’s outlet is vertically long and descends below the surface. I use the included filter cartridge to dampen the turbulence and to keep the shrimp from entering through the submerged portion of the outlet into the lift tube and descending under the gravel substrate when the power is turned off.