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Rataj and Horeman book

     Jerome, from Buenos Aires mentioned the Rataj and Horeman book and 
     asked about undergravel filters.
     This book is quite dated and the author held opinions about substrates 
     which should be taken with a grain of salt. He suggests that garden 
     soil, peat or clay should never be used in aquarium substrates and he 
     misleads many many aquatic gardeners by suggesting that oxygenating 
     the substrate by undergravel filters is important. I consider this 
     poor advise since there are great advantages to using substrate 
     additives such as small amounts of: laterite, clay, soil or peat. I 
     also believe that plants are the best method for controlling the 
     oxygen content (or more properly redox potential) of the substrate.
     One must understand the precautions for using such substrates. 
     Substrates should not contain too much organic material (consider 5% 
     optimal) and you do need to have a healthy initial planting with all 
     of the other conditions necessary for good growth otherwise your 
     substrate could foul. Under good conditions, plants grow rapidly and 
     their roots spread through the substrate and provide oxygen and draw 
     nutrients from the substrate. While there are some benefits to 
     providing a larger proportion of nutrients or a moderately higher 
     concentration in the substrate, it is impossible to prevent some of 
     those nutrients from making their way into your aquarium water. If 
     you're careful, this won't become a problem but don't overdo iron 
     additions to your water or it will turn green.
     I don't see any reason to recommend to anyone constructing a plant 
     tank to use an undergravel filter (unless you intend to perform some 
     kind of controlled experiment with multiple tanks). Such devices 
     interfere with natural biological processes which occur in the 
     substrate below about 1/2" where the lack of oxygen allows anaerobic 
     bacteria to reduce iron compounds to the more soluble ferrous state. 
     This condition also favors availability of nitrogen, phosphorus and 
     manganese nutrients within the substrate.
     I prefer to use high light intensities in order to ensure that my 
     plants can produce sufficient oxygen for my fish and to keep the 
     substrate sweet. Under these conditions, you need to supply CO2 or 
     else you will get pH soaring to +10 because the plants will resort to 
     utilizing carbonates. You also tend to get nutrient deficiency 
     symptoms much quicker; but plants grow faster and bigger too.
     Under moderate lighting conditions (2wpg), one must ensure that the 
     organic material used is very, very well composted or you could have a 
     situation where the tank goes into oxygen deficit. That would sour 
     your substrate and cause problems for some fish which require high 
     oxygen concentrations. Note that many kinds of fish such as our Amazon 
     natives like catfish and neons normally live in lower oxygen 
     concentrations. Specific types of peat may be useable since the acidic 
     content will prevent decay.
     Under lower lighting conditions (1wpg or less), you could probably 
     construct a soil or peat tank which required no CO2 injection. You 
     probably will not have oxygen saturation in such a tank and plants 
     should grow well but slowly with minimal algae problems.
     I think its a wise precaution to use an iron supplement to the 
     substrate such as laterite (mostly iron oxides and hydroxides) or 
     micronized iron (which you can find in gardening stores).
     Probably the best main component (90%) of your substrate should be 
     sand upto grain size 2-3 mm. You need at least 2-3 mm for the top 
     layer in order to get cuttings to stay in place and to prevent finer 
     components like laterite, soil or clay from mixing into the water. 
     Finer sand seems to be a good choice for the lower layers.
     Steve P in Vancouver where Spring has Sprung!!