As above, R/O water has a lower PH, so would be ideal for fish that like a low PH (like Discus) but not for those that like a high PH (like lake malawi cichlids). The PH will vary depending on which fish you want to keep, test your tap water and get fish that would be appropriate for that PH (however, most common communtiy fish like neons, platys, mollys etc will be fine in anything from 6.5 to 7.5). If you do go with R/O water, remember that you shouldn't use 100% ro water, 50/50 is a good ration between RO and tap water. Filtration- for a tank that size, an external canister filter would be ideal.
These filters are highly regarded within this forum. Live plants are awesome, but i've never had experience with using dirt under the substrate. Again, the temp will vary depending on which fish you want to keep, but most commonly kept community fish will do fine in 25 - 26 degrees.Lighting- different plants will require different lighting levels, low light plants (like Ferns mosses and anubias) will only need low to medium lighting (which is about 1.5-2.5 watts per gallon). Whereas high light plants would prefer around 3.5 watts per gallon. There are two main type of bulbs, T5 and T8, T5's are more energy efficient and brighter, and are just better
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, T8's produce more heat and are not as energy efficient. The canopy on the tank will either need to have t8's or t5's, so check before you go and buy them. What tank is it by the way?
Plants will benefit from the use of a good quality plant fertiliser, and budget depending a Co2 system
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