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I run two high tech planted cichlid tanks. My filters are all Penguin HOBs for mechanical polishing only as I have never installed the biowheels nor inserted any bio media in my HOBs. I never believe there is a need for dedicated bio media or bio filters as long as there is good circulation of oxygenated water over substrate and surfaces where BB adhere to, with or without plants. Vendors will be glad to up-sell you biomedia and bio filter when all you need is to provide the right conditions for BB to thrive. All aerobic surfaces in your tank (glass, substrate, rock, drift wood and plants) are in-situ bio media so why waste money and effort to set up bio filters. With plants, your need for dedicated biofiltration is even less as plants and BB both consume NH3 directly.

Since I have heavy bioload, my 75g is filtered with 2 Penquin 350, and my 125g with 3 Penquin 350 plus a wave maker in each to assist in circulation. This is in different from the zero tech shrimp bowls I posted above which I haven’t found the need for any circulation or filtration due to de minimus bio load.
 

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The only time I've had oxygen level problems in my Walstad tanks is during power failures. With both circulation AND lighting shut off, fish in my somewhat crowded tank show distress in about 48 hours.
When Hurricane Sandy knocked out my power for 11 days and basically destroyed the area I lived in, (house got down to the 40s/50s in temps) my fish in my heavily planted tank were wiped out in a day. My fish in my totally Un planted 20 gal high tank all survived even with the 25 degree drop from normal conditions that lasted over a week. That broke my heart from the hobby for several years. Battery powered pumps were unavailable during the event and I hadn't planned properly.
 

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There is going to be an article about oxygen levels in primarily no-tech tanks released in an online Czech magazine in the coming weeks, I will ask the author if I could share his measurements here sooner (including some "standard" and high tech tanks). Results are quite interesting (I have access to the preliminary version with raw data/notes). In the "worst" case I'll translate the final version of the article once it's made public.
Article can be found here http://e-akvarium.cz/casopis/akvarium48.pdf, but it's in Czech only (pages 28-49). I will try to summarize the most interesting parts (with some additional comments):

1. no-tech tanks, with soil substrate and plenty of plants (vallisneria, cryptocoryne,..) and low light have a very high difference between day and night, ranging from about 2mg/l to 8mg/l.
2. no-tech tanks, with soil substrate and plenty of plants and medium light have a very high difference between day and night, but! ranging from about 6mg/l to 12mg/l (hyperoxia at ~140% saturation).
3. no-tech tanks, with soil substrate and medium/plenty of plants and natural light are extremely depending on the day of the year (lenght of the day) and weather (sunny vs. cloudy) - 1mg/l to 18mg/l (!)
4. low-tech tanks, with soil substrate and plenty of plants and low light and with a filter - just a pure water circulation increases amount of DO by about 2-3mg/l
5. low-tech tanks with air driven sponge filters - they are at about 100% saturation, the amount of plants, light intensity or substrate seems to be insignificant

Author's conclusion (Jirka Scobak - @Jirka from Bratislava) is that "no tech" tanks can easily beat "low tech" tanks in the amount of dissolved oxygen. While I fully agree with this, at the same time we can't omit the fact, that they can be running extremely low on DO and without a very expensive DO meter we can't be really sure how good our tanks run. It's up to us to decide if we prefer a "safe bet" with stable, but a bit lower DO levels (water circulation/filter), or take a leap of faith and keep it "no tech".

Some of the repeated measurements allowed me to create this chart, showing minimum, maximum and average DO. They were NOT taken during one day, some of measurements are weeks apart, but they show how variable "no tech" tanks can be.



Edit: additional information can be found in this table. In comments you can find what kind of impact on DO have various actions (addition of light, change of the bulb etc.)
https://docs.google.com/spreadsheets/d/1VABiQEzIH3BcmUUlPNMi9BdKwMy_eTw-lKHLxNDxq48/edit?usp=sharing
 

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Do you know what's the optimal DO saturation for fish?
There is no single and easy answer. When I was doing research of existing studies, I stumbled upon many interesting articles. Some fish require at least ~90% saturation (mostly cold water species living in streams), others are able to live and breed even with 20% saturation (tropical species from Amazon). It mostly depends on natural habitat of fish - if they live in waters with very variable DO content, they have several biological and behavioral mechanisms to help them to cope with this situation. However in most studies it was mentioned that lower than optimal DO levels should be avoided. Long term hypoxia can result in sick fish, stunted growth, no/low breeding, birth deformities etc. Interestingly, high levels of DO (~150% saturation and higher) can have detrimental effects on fish as well. But again it depends on species, some cope with oversaturation better than others.

In the article above it's mentioned that Jirka successfully breeds stable colonies of livebearers (Girardinus falcatus and Heterandria formosa) in tanks which had the lowest extremes of 1ppm and 2ppm of DO. Fish don't show any visual impact of these less than optimal conditions. Maybe they don't reach their full growth/lifespan potential, but that is hard to confirm in home conditions.
 

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Here is a picture of my custom filter system I built for my 40G tank. It's a pretty simple sponge filter with a large area of medium poret sponge purchased from Swiss Tropicals (The makers of Matten filters). The filter will be driven by a water pump by day. You can see that the output spouts are well below the water surface so as not to disturb the surface too much. Surface agitation releases CO2 that plants need/use when the lights are on during the day. We want the CO2 to stay in the water so the plants can use it and in turn, produce oxygen. So no need to add oxygen during the day. Then, At night, when the lights are off and the plants are no longer producing oxygen, my filter will switch over to the air lifter to circulate the water and add oxygen to the water. Simple timers will turn each on and off as needed. Again, I refer to Diana Walstads book for my education on these issues. I hope I understand this correctly. Water pump by day, air driven by night. As per the original question of this thread (it was me) I understand that as my tank becomes well established, I may not need so much filtration and will dial it back considerably as needed.
 

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