fish tank maintenance

Question
Dear Lauren,
          For the last 3 years I have owned various types of tropical fish. Currently, I have 9 tanks in total ranging from 1 to 120 gallons & approximately 400 fish in total. Generally it's been quite good, except for the occasional serious problem here & there. However, most of the knowledge I have has been basically acquired from a combination of trial/error, & mostly my own observations over time. The problem I have is that perhaps, I've spoken to too many people regarding tank maintenance & general fish keeping. The variation between what I've been told is truly mind boggling to the point that I honestly don't know who to believe (despite the fact that almost everyone I've spoken with is supposedly "professional") hence the self taught side of things.
         What I'm really concerned with is the maintenance side of things- such as ammonia, nitrate & nitrite levels etc. Despite the fact that nearly all of my tanks are doing satisfactory I've never really been given any sound advice on this topic. Having observed my fish quite intensely over the last 3 years I've noticed that most if not ALL my fish are not as happy as they once were. For instance, I've got quite a few Guppies that I've bred myself using quality breeding stock from various places. I've also strictly avoided any inbreeding to take place & ALWAYS ensure that my breeding stock is of different "blood line" to each other as I find this the best way to yield quality offspring. Over the months however I've noticed more & more Guppies not living a "full term" of life which I believe to be approx 2 years or so. This is also starting to become the case with other species leading me to the belief that something is drastically not right. I'm 99.9ertain that my water chemistry is to blame as more often than not I'm measuring RIDICULOUS levels of the above mentioned elements even though a 20 water change is carried out every 10 days with vigorous gravel vacuuming as well. In a nutshell, despite my vigilant schedule with keeping up this practice there's no visible improvement in the levels measured as if to indicate that I'm fighting a "losing" battle. There was also a time whereby algae used to grow readily, now, I honestly can't even remember the last time I've seen some.
      I also have a few Flowerhorns & Red Devils on the other hand that don't exhibit any symptoms even though their tanks ALWAYS register the highest readings (from what I've found out these types of fish are slightly more resistant to these conditions than others hence their ability to put up with toxicity this high I presume). In any event, & in laymens terms WHAT is the best remedy to solve my very toxic problems.
       In closing, I'll briefly fill you in what I'm using (chemical wise) with every water change, as well as basic tank set-ups.

       For every 5 gallons of tap water I also add;
  * 1 Teaspoon of Aquarium salt
  * PH Adjuster if any is needed (depending on tap water conditions)
  * Required amount of SEACHEM Prime (to remove any Chlorine or Chloramine)

       What I really need is a recommendation of some SIMPLE but EFFECTIVE procedures/additives that will deem any water changes I carry out successful, without spending time & money on useless things. Also, could you kindly advise me on what sort of a role any "live" plants play, ie, would I benefit from adding something like this?
       I'm also using good quality & most importantly the correct size/type of filtration units which includes things like Carbon/Zeolite filters & a substance known here as Purigen which although very expensive, works very similar to active carbon inserts only 2-3 times more efficient. This product can also be re-generated when its absorbing ability has diminished. I have found this product to be MOST satisfying as it truly does a remarkable job but could it be affecting something else???
      Please forward any suggestions/advice via email & most importantly, thank you for your time.

Regards
Yiannis Lagos

P.S. Please accept my apologies for the "essay" type question- I just want to familiarize you with my current situation in the hope that you can help me  ;-)

Answer
Ok, Hello first of, Im not quite sure what im answering here but ill try my best.

A Plant called Elodea
Which looks like this...
http://www.vcbio.science.ru.nl/public/Final-Images/PL_Final512z_301-350/PL0330_5...
Is a oxygenating plant, in some fish stores it is even called oxygenating plants, also it is very good for fish who lay eggs, if you are breeding them.

I use a chemical called aqua-safe for my tanks, and i do not pu aqauarium salt in, but i know you can.
I use something called stress zyme that reduces stress but also put good bacteria into the water, some others i know use something called cycle, which does the same thing.


You could have too many fish in your tanks, you could be adding fish too soon, or you could be over feeding.

Also do you change the sponges in your filters monthly?
As you are supposed too.


Please do ask a follow up question if you would like more information or i haven't answered some questions