I am considering to augment the power from my solar cell with a dc adapter whom will take up the slack when the sun can not provide, and feed it to my solar charger/regulator and led-acid setup. Will a diode to prevent back feeding to my solar cells be enough or do I need some thing else? I am thinking that the dc supply will act as a second solar unit in parallel, am I wrong? The reason for this setup is a 12v circulation pump and led light for my fish tank, with a battery backup. The heater will have to remain ac for now. Hope you can help.
Yes, a diode is what you need, preferably a Schottky since presumably the voltages are low.
You do have to pick a DC power supply with just the right voltage. This should be the lowest voltage your solar charger/regulator can handle. You want the power to come from the solar cell when it has sufficient illumination, so the power supply voltage needs to be lower.
Personally, since you said you're using a PV solar, PLUS battery that you want to function as primary power, with the dc power converter/supply as a "second backup" to the battery, it might be a good idea to hook up the PV panel+battery (with protection diodes between the PV panel and the batt. always) on one "subsystem," with a "low voltage trigger" set at ~11.5V that "turns on" the DC power supply until the battery voltage reaches back up to a 12V "cut-out". This would be easy to implement with either a voltage reference & comparator, or a microcontroller, & a transistor to "turn on/off" the DC back-up source.
Alternately, here's one circuit you could build from discrete components, which would allow the DC power source to 'kick in' to stabilize load voltage whenever Vbatt drops below about 11.5V. This circuit doesn't include any hysterysis, so it won't meaningfully charge the battery & (unless you substitute a comparator/schmitt trigger for Q1) will keep the P-MOSFET in its 'linear region' much of the time, so it'll need a heatsink to dissipate the energy from the voltage drop.
(You could also, depending on your specific requirements, get rid of Q1, Q2 & all of the resistors, then connect the Gate of Q3 to the Vbatt+ trace (before the 'output' shottky diode) & use a P-MOSFET with Vgs(th) suitable to cause it to 'bring online' the DC supply @ your chosen voltage differential.