Low Tech Planted Tank - Advice needed

Well of course we need a current pic of the tank!
Looks good even 2 weeks ago though! needs some sort of carpet in the front fading into the back left. Pearlweed is a low tech carpeting plant you may want to look into.

 

The only time water changes are harmful is when they cause big changes in how much CO2 is in the water. Tap water tends to have quite a lot of dissolved CO2, so each 50% water change causes a spike in the CO2 in the water. That invites black brush algae (BBA) to start growing. But, if you also dose Seachem Excel daily at 1 to 2 ml per 10 gallons of water, that will greatly reduce the threat of BBA. Water changes don't adversely affect the fish and usually stimulate them. Not doing water changes can lead to high TDS, brownish colored water, and, when you are fertilizing the water, excessive levels of nutrients in the water.

I'm not familiar with that light fixture you are using, and I don't see any PAR data for it, so I don't know if you have too much, too little, or just right light intensity. The light intensity, more than anything else, is what determines how much fertilizing, etc. you should be doing. And, fertilizing means regular water changes are a good idea.

 

I have had that problem a few times. I think it depends a lot on how much light you have. When I have had the problem, as I recall, I had close to medium light instead of low light.

 

I am no expert but my advise...

I dont know a lot about pearlweed, but stauro repens is super easy. Does well in low tech tanks, no need for co2. You can make a nice carpet of that to boost your plant biomass, and have a beautiful carpet.
Srepens is a short stem plant, you can top and replant, and the topped plants will grow side shoots and maybe runners?
Have you read Tom's original article on non Co2 method? There is a lot of good info there
Don't buy liquid ferts, those are 98% water. Buy Kno3 and Kh2po4 from http://www.aquariumfertilizer.com/ or http://greenleafaquariums.com/
these ferts will last for a LONG time, and they are dirt cheap compared to name brand liquid ferts. Tom suggests using Seachem equilibrium in a non co2, no water change tank.
Over time the plants will consume the calcium and magnesium, lowering the hardness of the water, so you can keep from bottoming out of Ca or Mg with Equilibrium.
Perhaps if your tap water is really hard, and high in probably Ca, you could just get away with adding a little epsom salt with your top offs, not sure about this though.
If your light is too strong, just decrease the photoperiod... I have had to do this in my non co2 tank because green spot algae grew like wildfire

This is from Tom's non Co2 method article about fertilizing...

While trace mixes can be added, I decided to use SeaChem Equlibrium instead.
It has Fe and Mn as well as Ca/K/Mg/SO4.
I will add about 1/4 teaspoon per 20 gal tank once every week or two.
This greatly enhances the growth of the plants.
I also will add about 1/8" and 1/32" teaspoon of KNO3 and KH2PO4 respectively once a week or two.

 

I'd never advocate zero water changes, it's not good for the health of your critters, even with high density plant biomass and efficient filtration. Personally I'd keep up with the 50% water change and use it as an opportunity to remove dissolved and solid organic compounds. But 20-30% once a week should be fine.

Fertz dosing is always a good idea even with a nutrient rich substrate. Most plants will uptake nutrients through leaves as well as roots so it's kind of good horticultural sense to feed both sites. Just use a reduced dose as posted above by Dale, or you could use 1/5 - 1/10 dose of a ready mixed product, it'll still be reasonably economical at the lower dose rates needed.

I'm very sceptical about the whole CO2 large water changes and algae hypothesis...either way, I believe that if a tank is well balanced it'll be robust enough to withstand frequent and substantial water changes without incident. High plant biomass always helps with this.