Hair Algae

[Akasha72]

my tap water is very soft coming out of the tap at around 7.4 with zero kH and gH. The po4 reads at 1.6ppm straight from the tap so my water is not good. Thankfully each time I've tested nitrate from the tap it's always been zero

 

[StevenF]

 
 
my tap water is very soft coming out of the tap at around 7.4 with zero kH and gH.

GH is primarily a measure of Alkali earth elements in the water.  Magnesium and Calcium are the most common. 
KH is primarily a measure of  Alkali element (soldium and potassium) and alkali earth elements.  
 
If you measure zero there isn't enough of these elements to register.  but just enough to creat a PH of 7.4.  RO water has zero KH and GH and a PH of 5 or 6.  Your water is almost as bad as RO water.  You might also be short on several trace elements but detailed  lab tests would be needed to confirm that.  You are lucky that your tank has done as well as it has.   
 
When you add your tap water to the tank in about 2 days the plants will have likely removed all of the calcium and magnesium and potassium from the water.  After that there is essentially no growth untill the next water change  PH will also drop during this period.   During that no growth period Nitrates and phosphates will not be extracted from the water and they will build up.   The buildup of nitrates and phosphates will help push ph even lower.  KH levels are not sufficient to prevent larg fast swings in PH.  
 
Adding 2 or 3 decorative snail shells would stabilze KH  and PH at about . It would also reduce Stress levels in your fish and and reduce the occasional plant loss.  Adding a trace fertilizer would also help your plants.  However it may not be possible to add enough potassium to remove all of the nitrate or phosphate.  The primary concern with potassium is that it tends to push ph up toward 8 or more which is not desirable.

 

[Akasha72]

thank you Steven, that makes sense. I add Easylife Profito fertilizer daily (taking the weekly recommended dose and dividing it by 7) and also Easycarbo too. The Profito doesn't give me a list of nutrients but it does say it doesn't contain phosphate or nitrate which is why I chose this fert - I didn't want to add anything I already have in abundance.
 
I do run some ocean rock in my filters to help the pH but that help is minimal. I'll have a look and see if I can get a decorative shell of some description. As things are all my stock are amazonian (apart from my SAE's which I got to eat the BBA) so they are perfectly happy and thriving in the water I have 

 

[StevenF]

 
 
 I add Easylife Profito fertilizer daily (taking the weekly recommended dose and dividing it by 7) and also Easycarbo too. The Profito doesn't give me a list of nutrients but it does say it doesn't contain phosphate or nitrate which is why I chose this fert - I didn't want to add anything I already have in abundance.

I personally will not buy any fertilizer that doesn't say which nutrients it covers.  Some are intended for use with tap water and therefore mainly have NPK.  Others may say no nitrate or phosphate  on the front.  But on the back then list it in ithe ingredients. Some manufactures assumes the tap water delivers all the micro elements plants need.  
 
The first fertilizer I purchased had everything but copper.  With RO water I had no copper and as a result the plants didn't grow but he algae did grow very well.  I then switched to Seachem Flourish comprehensive which worked much better.  However I have recently started using SeaChem trace.  t covers all the micro nutruents except iron, Calcium and magnesium and does not have NPK..  Due to years of crushing pond snails I have plenty of Calcium and magnesium so I don't  need a fertilizer with them.  For Iron I am mixing the trace with Seachem iron.  I can now dose the tank with NPK  separately if I want.  So you might want to consider seaChem trace plus iron.  And if you want you can experiment with  adding a small amount of  potassium to the mix.
 
I listed decorative snail or Sea shells because they are cheep in the fish stores and you don't need any.   Replace them as needed as they slowly erode away.   You can put them in the filter if you don't want to see them.  However you could also use crushed coral  but that is often sold by the pound.  which is way more than you need.