you need to cycle a tank before you have anything in there.
cycling a tank is the process of adding LR and sand to a tank and letting it sit. you NEED to let the tank sit for a good deal of time SOOOOOO that the tank builds up a population of bacteria that breaks down all the nasties in the water.
you will need a quality test kit to monitor this process. what happens is you set up a tank with Lr 1-2 lbs per gallon. this LR is "live because it already has some of the bacteria on it.
when you set up a tank, the die off of critters in the rock, or the addition of a shrimp or somethin causes an ammonia spike.
then your tank develops a strong population of ammonia eating bacteria, that poops nitrite.
then you have a nitrite spike, which triggers the develpoment of nitrite eating bacteria that poop nitrate.
nitrates are expelled from your tank via bubbles, skimmers and water changes.
during a cycle, you will be able to see the 3 spikes. Ammonia, then Nitrite, and Finally Nitrate, which means you are ready for a water change.
once Ammonia, Nitrite are reading zeros. and Nitrate is <10ppm, your tank is cycled.
at this point you will have an abundance of algeas, diatoms and other things you, dont really want.
its time for a CUC (clean up crew.)
these are snails, hermits, and shrimp etc. once you have completed a cycle, its time for these guys to go to work.
you need to be careful when you start adding livestock, because each little creature you add, changes your bio-load. soo your bacteria need time to adjust to the new demands. so add life slowly
then for a 20 gal you can have like 1-2 small fish, a cuc, and corals.
corals.
unless you're gonna be satisfied with just things like mushrooma and star polyps, you will really need to up your lighting.
from personal experience, id suggest the corallife 96watt Pc fixture. its gonna cost you around 100 bucks, but then you can keep like tight shiz in your tank.
as far as petco LR goes, id not suggest it. petco sux. and id assume their LR does too.
if you get on reef central and go to either the local club forum for your state, or the livestock selling forum, you can pick up really nice Lr from a person who is breaking down a tank for like 2-4 bucks a pound.
FLow.
i have an 18 gallon tall tank. for flow i have 2 rio 90s, a nano korilla, and a 250gph hob ghetto skimmer thing (so @ 670 gph thats 37.22 times turnover per hour.)
you currently have about 200gph. id suggest you up that substantialy up yer flow. flow is important because its forcing water against you sand bed and LR, which is your filter.
more flow = more exposed surface area which means your system is more effective/efficient at removing funk.
your fish....
sorry man, but id have to suggest taking back all your fish. you cannot keep a mono, period. they grow real fast and in 6 months it will be too big for a 20 gallon tank.
the GSP aint reef friendly. its liable to kill all your inverts in a reef tank.
and damsels suck. you cant have anything in your tank during a cycle, and once its done, a damsel will beat up anything else you want. i recently just spen 6month trying to get a damsel out of my tank. i had to build a trap.
click the link in my sig to see my 18 gallon. its officially 1 year old.
no one is accusing you of doing anything wrong,because just like me last august, no ones told you the truth.
this takes time. if you want a tank to be set up by thangiving, salt water is not the way to go.
nothing happens fast in this hobby. if you rush it, you are garunteed to fail.
the more patience you have with it, and the longer you take, the greater the rewards will be.
SOOOOO. if you want to go salt/marine. you need...
to take your fish back.
get rid of the fake stuff. resiens can leach bad stuff into your tank.
find some LR, sand, and a light. BUYa test kit, refractometer, and find Ro/Di water (buy a filter or go to king soopers)
id suggest what you have + 2 nano korillas (powerheads)
and the other lil stuff YOU personally want.