Sorry for the long winded post:
Since I first asked this question, I have since begun RO top off on both of these tanks but i did not try to maintain kh with baking soda in either of them.
The results have been interesting. .. the 5 gallon has since stabilized with gh 25, kh 1, and ph 6.8, with seachem fertz and equilibrium but no baking soda. A thriving RCS colony, 3 amanos and many ramshorn snails have populated the tank since August and I just added 6 cardinia babaulti green... the kh and ph have remained constant all this time and nitrates are almost non existent with around 60% surfsce coverage by frogbit, salvinia natans and watersprite complementary to the mosses and crypts below. ..
My other 15 gallon tank has seen a much different result... the hardness stabilized after switching to RO topoff but the kh began to drop down to 7 in September. .
By the end of September I felt the hardness was still too high so over a period of several weeks I changed 50% of the water for straight RO water with a result of gh 16, kh 4 and ph of 7.5 (down from 8.2), I also added 7 juvenile yellow forktail rainbow fish at this time...
By mid November my ph seemed to fluctuate and my amano shrimp started to get sluggish, turn opaque and by December my kh was less then 2 and I feared my ph was crashing... the amanos started to die and at least 3 CPDs were missing besides the one jumper and one very sick one that I euthanized...
At this time I gave in and added baking soda, enough to increase kh by 1dh at a time... more shrimp died... These fluctuations were no good I determined... kh 5 and ph of 7.8, then you to 8.0 again
This week I added enough baking soda to increase kh by 1/2dh when it fell again to less than 3 and ph fluctuated to 7.5
So as it stands I don't know if this is a simple kh buffering problem and I should or shouldn't be adding buffers to maintain a certain level or whether I have an underlying problem with too much organic acids driving my kh down... maybe my bioload is too high or feeding too much? My nitrates are 80+ppm with no other spikes that I know of and with 7 healthy yellow Forktails and 7 CPDs I don't think the bioload is too high for such a heavily planted tank so I am at a loss... There was even an unknown fish fry seen yesterday so it can't be that bad for the fish if they are breeding
So either I have to make a big water change or add some emersed house plants to suck out the nitrates...