See
this post for more detailed contents of bleach and chlorinating liquid. As was noted above, the rest is mostly salt water.
First of all, Trichlor tabs are not 98% chlorine, but rather 98% purity for Trichlor itself. There is, however, a "% Available Chlorine" number and for Trichlor that is typically 90%-92%. The "% Available Chlorine" does
not mean a weight percent of chlorine atoms themselves. This is because 100% is defined as that for chlorine gas that has 2 atoms only one of which becomes chlorine in water (i.e. hypochlorous acid) the other becoming chloride (hydrochloric acid). If one had a chlorine compound that was composed solely of chlorine atoms all of which produced hypochlorous acid in water, then that compound would have 200% Available Chlorine so you can see how the definition of available chlorine can be misleading.
So Trichlor is actually closer to half its weight (45%-46%) in chlorine atoms with the other half being a Cyanuric Acid (CYA) base, though they are bound together in a single chemical (see the
Trichlor chemical structure). You can see from
this post that just because the chlorine is more concentrated in Trichlor, it doesn't mean it is less expensive. Bleach and chlorinating liquid are very cheap by weight or volume because they are mostly water. So while getting Trichlor is a denser source of chlorine, it is not cheaper for the amount of chlorine you get after accounting for other pH and TA balancing chemicals you also need to add. Also, it has the side effect as indicated below in these rules that are independent of concentration of product or of pool size:
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.
Typical chlorine consumption in a residential outdoor pool exposed to full sunlight is often around 2 ppm FC per day or more. At 2 ppm FC from Trichlor, one would be increasing CYA by over 35 ppm per month if there were no water dilution so you can see that the CYA will build up quickly making the chlorine less effective and potentially leading to algae growth unless the FC level is raised proportionately with the rising CYA level. It is this phenomenon that explains why some pools using Trichlor start out fine at the early to middle part of the season, but run into problems in August or September after the CYA has continued to build up. In other situations, it explains why there are no problems for the first year or two but then there are issues. It also explains why larger pools or those with cartridge filters are more likely to have problems due to less water dilution than smaller pools with sand filters that require regular backwashing that result in greater water dilution. The specifics of each pool depend on dilution rates and what happens over the winter as well as whether algaecides, phosphate removers, or other products are being used and also whether there is regular shocking that tries to make up for what is essentially too low an FC/CYA ratio due to the CYA level that has become too high.
