Liquid fertilizer and substrate (root tabs & sticks) fertilizer are two very different things. Substrate fertilizers only feed substrate rooted plants (swords, crypts, vallisneria, aponegeton...) and are useless to floating plants, plants that root on rock and wood (Java Fern, Moss, Anubias) and marginally useful to stem plants as these take nutrients from the water via roots along the stems and leaves. Liquid fertilizer will benefit all plants, since nutrients come from the water and are taken up by the roots, and in substrate rooted plants the water passes through the substrate (if it is healthy) and thus gets to the roots. Swords and crypts are heavy feeders, so direct substrate ferts help these, but otherwise liquid works fine. Which is one reason I never bother with enriched substrates, it has limited practical value for the work and fuss.
Seachem makes several plant products as you know. One of them is called "Flourish Comprehensive Supplement for the Planted Aquarium" and this is a basic comprehensive fert, although designed to be used with some of their other products. I used this one alone for more than a year with very good results. Fertilizers are somewhat Dependant upon your tap water which may have some minerals in it, and some minerals occur in fish foods. Another good comprehensive fert is Kent Freshwater Supplement. I used this for more than 12 years.
Re the 180g, as you are thinking along the lines of T8 light I would aim for 1 watt per gallon, and have the tubes distributed so that the entire length of the tank is covered. Before doing this, I would check to see what lengths the tubes come in. For example, you can buy very inexpensive full spectrum tubes made by Phillips and Sylvania and others from hardware stores like Home Depot, but I'm not sure if they come in various sizes. I have seen them in 48-inch tubes, but not smaller in my local HD. As 48-inch is a very common and standard length, it might be advisable to use this and be assured of having an open filed on types of tubes.
Last on the Hornwort, it may be the light. Daylight is probably entering the tank from the front, and supplementing the light requirement of hornwort. I certainly would not change the background; black or a dark background does wonders for the plant and fish colours, and there is the issue of light effect on the fish that I've written of elsewhere.
Byron.