I think what happened is that your tank was not cycled at all. Bottled cycling products often do not work very well. And the test values you needed to see were ammonia at zero, nitrites at zero and nitrates at some level between 20 and 30 ppm
Also, what do you mean exactly by purified water ? I know you are using dechlorinator, but on what kind of water ? Tap, bottled spring, Reverse Osmosis, [ RO
], Distilled ? What brand of dechlorinator do yo use ?
But because the cycle hadn't happened yet, when you added fish, you started a cycle, and now you have too much ammonia and nitrite and while the nitrate level is not yet toxic, up to 50ppm is considered safe, the other two toxins are killing the fishes. I feel quite badly for you, as you clearly tried to do it right.
But you can't depend on bottled bacteria alone, and even if they had worked, they would still need to be fed. They only eat ammonia and nitrite.. so you'd have had to be feeding them all that time with pure ammonia or by adding just a few hardy fish, or possibly be adding some raw shrimp, which would decay and produce some ammonia, but all that is moot now.
Because now you have fish.. so first, do not add any more fish until the cycle is stable. Start doing water changes until the ammonia and nitrite readings come down to .25 or less. Big water changes are much less stressful than the toxins are, so do as many as needed to get the levels down. You can do half a tank at a time, or a third at a time, as often as need be to get the results you need.
Test often and do a water change every time those two levels exceed .25. You already have nitrates so you already have both the types of bacteria you need for a cycle, you just need to get them balanced so they are able to handle the load of fish you have. The bacteria reproduce about once every 24 hours or so, so it might take a week, or longer, to get it under control.
It is unfortunate, but the other fish that have been exposed to the ammonia and nitrite may not live very long. It burns gills and can cause other damage, and it's not reversible. Some fish live longer, some don't live very long at all.
Catfish are quite sensitive, being 'scaleless'. They aren't really without scales but the scales are very fine and small, making them more sensitive to any toxin. Most of the dwarf gouramis are also quite fragile and sensitive and may die for no apparent reason.. at least in this case you know why they died. They prefer quiet water, plenty of plants both planted and floating and no boisterous tank mates. Perhaps not the best choice for community tanks. Always pays to research the needs of fish you are interested in keeping, as the fish stores don't always get it right. Just because a fish is peaceful doesn't automatically make it a good all around community fish.
Once you get the cycle under control, with zero ammonia, zero nitrite and nitrate around 20 ppm
, then you can consider adding other fish, but only a few at a time. Each new fish adds a new load of ammonia, which the BB, beneficial bacteria, need some time to adapt to, that is, grow enough of themselves to consume and convert it.
Also, if you only have lights on 3 hours a day, any live plant you have is probably not going to do very well. Even low light plants need light for about 8 hours a day. Algae is inevitable, but low lighting levels for low light plants usually won't cause much of an algae issue. Many algaes show up as a result of nutrient imbalances, rather than simply an excess of light. Brown algae [ diatoms] often appear in new tanks, but it mostly goes away on it's own once the initial supply of silicates is used up by the diatoms. If you don't want to deal with live plants, go all fake instead.
And why salt ? its not needed unless fish are sick or are brackish species, like mollies. But even mollies are quite adaptable to fresh water and breed readily in it, so salt is just not necessary. Many people use it because they think it has some sort of tonic effect but it's a myth. It's useful for treating some illnesses and wounds but as a regular additive, it's not good. Catfish in particular do not tolerate salt for any length of time, another reason your pleco may have gone earlier than others. Cories don't tolerate it either, so stop adding salt.
Last, the Nitra zorb.. it can only absorb so much, and it's not the way to handle the problem long term. Long term you want a cycled filter that has enough biomedia in it to support good colonies of nitrifying bacteria. Filter floss is great for water polishing, or keeping it very clear. It gets fine particulates out. You maintain filters by squeezing or rinsing out the sponge or other media in tank water, from a water change, or in dechlorinated water, so as not to kill off BB. Once clean, you replace the media in the filter. Even floss can be reused many, many times before it's so ragged it's falling apart, though you can replace it more often if you wish.
Ceramic media, in a mesh bag, is another good bio media that lasts almost forever, and sponge will last many, many years before it begins to degrade. The cartridges sold to refill filters are expensive and not any better than basic media, and most of them have carbon, which is also not needed. Carbon is useful for removing some medications, odours or discolouration, such as from wood, from water. Otherwise, it's costly and serves little purpose and must be replaced often to maintain freshness and activity.
You can buy filter sponge and cut it to fit the space. Floss is the cheapest media of all, and many ceramic types or bio balls can be bought also, and kept in a mesh bag so they stay together. All will provide large surface areas for BB to colonize.