New member with stain problems

[waterbear]

I am familiar with the walmart kit directions...I have one....OTO is a test for Total chlorine. It will not accurately test FC.
Light brown stain is probably iron. The fact that ascorbic acid removed it easily strenghtens my belief that your stain is from iron.
As far as keeping the stains from coming back it's very simple. Watch your pH (staining usually happens when the pH rises too high) and keep a seqesterant in the water. There are many good ones out there. I have used the Jack's Magic line (Pink stuff for iron, Blue stuff for copper or combinations), ProTeam Metal Magic, and NaturalChemistry's Metal Free with good results but there are other good ones also. It is important to follow the directions in terms of water balance before applying. (some like the pH to be on the high side and some might want the chlorine level droppped a bit). The initial treatment dosage might mess up your water balance by lowering pH and eating up your total alkalinity but just rebalance the water after 20 hours or how ever long the manufacturer suggests. After that just keep up with the maintenance dose weekly or every two weeks and you should have no problems! If the metals are coming from your fill water then it's a good idea to add a little extra each time you add water to your pool.

I am guessing from your numbers that you have recently treated with ascorbic acid since your chlorine levels are very low and your alkalinity is low also. If there are no stains then add a sequesterant, filter for a day or so, and then slowly raise your chlorine level and adjust the TA and then the pH. Try not to let the pH climb above 7.6 to help avoid staining in the future.
Since your CYA is still on the low side you can continue using the trichlor tabs to get it up a bit higher. You then might want to consider switching to bleach as most of the regulars on the forum use it as their main source of chlorine!