Your tank last year looked very nice, sorry to hear the plants all died. I think I know why (in part anyway) and I'll exlain in a moment. My two acquaria have been running as you see them for over six months now; I moved them last autumn, but I have maintained tanks like these for almost 15 years, so it is possible to have a balanced planted aquarium over the long term without a lot of expense. I still have some of the plants from 12+ years ago--that large Echinodorus macrophylus that is just to the right of the centre in the 90g with long stems and large leaves at or near the surface is one such plant. This is the fourth set-up it has been in, but it is still vigorous and thriving.
All of the plants (except for the Java Fern
) in the photo of your tank are what are termed bunch or stem plants, as opposed to rooted plants. The plants in my two aquaria are all rooted plants, except for the Brizilian Pennywort that is growing up the two sides of the 90g, that is also a stem plant. Rooted plants tend to grow slower than stem plants, and therefore are better suited to a low-tech aquarium without CO2 addition. If I wanted to have real success with stem plants, I would find it very difficult without adding more light and then probably CO2 and a substrate designed for plants with added nutrients. So the first thing you need to decide is just what sort of planted aquarium you want in the end, and then work towards achieving that goal. This is where we get into low-tech and high-tech systems. The latter is obviously more expensive but it also requires considerably more maintenance each week (with stem plants).
First, I'll explain a bit about the plant differences. Aquatic plants vary from terrestrial in the root systems and the leaves. Roots of aquatic plants are needed to anchor the plant and to collect and store nutrients, and they also release oxygen into the substrate which is important for the aerobic bacteria that live there. Terrestrial plants also need to collect water which is why they have fine hairs that are absent in aquatic plants. The leaves of acquatic plants are thinner (do not have a thick waxy outer layer like land plants) so that liquid passes through more easily which helps the plant to take up nutrients through the leaves, which are also used to collect light for photosynthesis. Gasses are also exchanged through the leaves of aquatic plants as well as the roots.
Stem plants have no base root system, but the roots develop along the stem as you probably noticed with your plants. Rooted plants do not (normally) produce roots from the leaves, but rely on a more developed base root system. Most of the stem plants are fast growing compared to the slower-growing rooted plants. This is why the stem plants require more light, more nutrients and more CO2 than is usually available in a normal aquarium. As you probably know, when fish respirate their gills remove oxygen from the water and expel carbon dioxide (just as we do when we breathe). The plants use that carbon dioxide for photosynthesis. But unless the aquarium is very heavily stocked with fish (and more than would be safe), the carbon dioxide will not be enough for the faster-growing stem plants and they will pale, loose leaves, and eventually die back. The rooted plants however, having a slower metabolism, can mange perfectly on the amount of carbon dioxide produced by the fish in a normally-stocked aquarium. But the light (both its level and duration) and the nutrients (trace elements, minerals, iron, etc) must be in balance to the available CO2, otherwise algae will over-run the aquarium as you have seen.
So, the reason in my view for your plants failing was that the CO2 was insufficient, and the light may have been as well (depending upon what you had at the time). You may have had too many or too few nutrients (your fertilizers you mention) to balance the light and CO2. Another point is the pH and hardness of the water, but I suspect this was not a factor because the plants you had in your tank would be fine in slightly acidic or slightly alkaline water (pH from a low of 6.5 to a high of 7.8 roughly). Guppies, mollies, platys and swordtails are livebearers that prefer water that is slightly alkaline [pH of 7.0 to 7.8] which is within the range of most water supplies [unless you're like me and live where the water is slightly acidic at 6.8]. If you haven't. you should test the pH of your tap water (and your aquarium) just in case, but I'll assume it is OK. Most of the common tetras and the pleco would prefer slightly acidic water but can be maintained in neutral to slightly alkaline water once acclimated, so that isn't a problem for you.
Now a word about low-tech and high-tech. High-tech means providing a substrate that is enriched with nutrients (either as an added layer of laterite or similar material, soil additives or the prepared plant substrates you can buy in the aquarium stores), CO2 injection daily with a diffuser system, light in the range of 3-4 watts of full spectrum per gallon, and (probably) daily dose of trace elemnt/mineral fertilizers. Stem plants will grow quickly given these conditions, and require weekly pruning to prevent them from covering the surface and blocking out light. Rooted plants will also grow a bit faster and may be bushier. The aquascape will be constantly changing becuase the plants grow so fast, so you'll be spending more time maintaining the tank.
Low-tech requires a minimum of 1 watt of full spectrum light per gallon (and can go up to 2 watts per gallon but no more) and adding a trace element fertilizer once or twice a week. The substrate can be regular aquarim gravel; using a nutrient additive will probably not harm anything, but as it is unnecessary why bother. Fertilizer tabs inserted into the substrate sometimes benefit heavy feeders like large sword plants, but I have had equally thriving planted aquaria with and without substrate fertilizer. I have to add liquid fertilizer twice a week; I have twice in the last six months experimented with only once a week, but in both cases the leaves of the swords started yellowing and becoming "transparent" so I now know that twice a week provides the nutrients to balance the available CO2 and light. I use Seachem's Flourish Comprehensive Plant Supplement; I have previously had equal success with Kent's Freshwater Plant fertilizer. All you need is the "basic" fertilizer with trace elements; there are many specific fertilizers made by both these companies, such as iron, potassium, nitrogen, and so forth, but those are not advised in a low-tech set-up because the light and CO2 won't balance them and they will not be successful. The light is all-important, as it must be the limiting factor for the plants. Once the light is balanced by the available CO2 and nutrients, algae will not be a problem as I've explained in a previous post.
This has been a lengthy post, but I hope it helps. You may have questions from the above, so fire away if you do. Byron.