Here is a question I get in every workshop: "The plants look fine, so why should I dump a reservoir of nutrients I just paid for?" Fair question. The honest answer is that a hydroponic reservoir is not a static tank. Plants drink water and specific nutrients at different rates, so what starts as a balanced solution slowly turns into a lopsided, drifting one, even while the leaves still look green.

Wait too long and you get lockout, pH swings, and salt buildup. Change it on a blind weekly calendar and you can waste good nutrients. The right answer sits between those two, and two cheap meters tell you exactly where. Here is the schedule I actually run.

Top Up or Full Change: What Is the Difference?

These are two separate jobs, and mixing them up is one of the common hydroponic mistakes I see most.

A top up replaces water that plants have transpired and evaporated. You do this every day or two, adding plain, pH-adjusted water, not more nutrients. As plants drink, the water level drops but a lot of the dissolved salts stay behind, so the solution gets more concentrated. Adding plain water brings it back down. If you top up with full-strength nutrients every time, salts pile up fast and EC climbs out of range.

A full change means dumping the entire reservoir, rinsing it, and mixing a fresh batch from scratch. This resets EC, pH, and the nutrient ratio all at once. Top ups buy you time between full changes; they do not replace them.

How Often Should I Change the Whole Reservoir?

My default is every 7 to 14 days for most leafy crops and houseplants in water. Small tanks and hungry plants land at the 7 day end. Large, well-buffered reservoirs with light feeders can stretch to 14.

Three factors pull that number around:

  1. Reservoir size per plant. A single lettuce in a 2 L jar exhausts its solution far faster than four plants sharing a 40 L tank do per litre. Small volumes drift quickly.
  2. Plant size and stage. A mature, fruiting plant transpires litres a day and strips nutrients hard. A seedling barely touches it.
  3. Water temperature. Warm solution above 22°C burns through dissolved oxygen and grows more microbes, so it spoils sooner. This ties directly into hydroponic water temperature and dissolved oxygen levels.

If you only take one habit from this article, make it this: mark a fill line on the tank. Once the level drops to roughly half through drinking and topping up, do a full change rather than topping up yet again. By that point the ratios have drifted too far to trust.

Which Numbers Tell Me the Solution Is Spent?

Stop guessing and measure. Two meters, both cheap, remove all the doubt.

EC (electrical conductivity) reads total dissolved salts, so it tells you how strong the solution is. Set a target for your crop, for example 1.2 to 1.8 EC for lettuce or 2.0 to 2.5 for fruiting tomatoes. Then watch which way it moves:

  • EC climbing above target: plants are drinking more water than nutrients, so salts are concentrating. Dilute with plain water.
  • EC falling below target: plants are stripping nutrients faster than water. The solution is running out of food.
  • EC drifting more than about 0.4 from target and refusing to settle: change the whole thing.

pH is the second gauge. A healthy reservoir holds in the 5.5 to 6.5 band. As nutrients are used up, pH tends to wander, and once it will not stay put after you adjust it, the buffering capacity is gone and it is time for fresh solution. My pH for hydroponics guide covers the adjusting side in detail.

The Signs You Left It Too Long

Even without a meter, the reservoir will tell you when it has turned. Watch for these:

  • A shift in colour or a cloudy, tea-like tint to the water.
  • A sour, swampy, or rotten smell. Clean solution smells of almost nothing.
  • Slimy biofilm on tank walls, pump, or roots.
  • Salt crust building up at the old water line or on net pots.
  • New growth showing deficiency signs (pale or yellowing young leaves, spotting) despite a full tank, which points to lockout rather than lack of food. My hydroponic nutrient deficiency guide helps you read those symptoms.

Any two of these together mean change it now, not on schedule.

A Simple Reservoir Routine That Works

Here is the exact rhythm I hand beginners. It fits a small deep water culture bucket or a Kratky-style tote just as well as a bigger system.

Daily or every other day: check the water level and top up with plain, pH-adjusted water to the fill line. Glance at the tank for smell and clarity.

Twice a week: take an EC and pH reading. Adjust pH back into 5.5 to 6.5 if needed. If EC has climbed, dilute; if it has crashed, note that a change is due soon.

Every 7 to 14 days, or when the tank hits the half-full mark: full change. Dump the old solution onto ornamental garden plants rather than down the drain, rinse the tank and air stone, wipe off any biofilm, then mix a fresh batch to your target EC and pH. Reset your calendar note.

Keep it that boring and consistent, and you avoid nearly every reservoir problem. For the whole system context, our complete hydroponic growing guide and the beginner plant list fit around this routine.

Frequently Asked Questions

Q: Can I just keep topping up with nutrients instead of ever doing a full change?

A: No. Topping up with nutrients steadily raises EC and lets the ratio of individual elements drift, because plants use nitrogen, calcium, and potassium at different speeds. You end up with a solution that reads strong on the meter but is short on whatever the plants stripped first. A periodic full change is the only reset.

Q: My EC is on target but pH keeps drifting every day. Is the solution bad?

A: Probably yes. Constant pH wandering, where it will not hold after you adjust it, usually means the buffering is exhausted or microbial activity is high. Do a full change, clean the tank, and check your water temperature, since warm reservoirs above 22°C spoil faster.

Q: How do I dispose of old nutrient solution?

A: Diluted, it makes decent fertiliser for outdoor ornamental beds or lawns, so pour it there rather than repeatedly down a drain. Do not tip it into natural waterways or storm drains. Never reuse an obviously fouled, smelly batch on the next hydroponic cycle.

Q: Does a bigger reservoir really need changing less often?

A: Yes, per plant. A larger volume dilutes the daily swings in EC and pH, so it drifts more slowly and buffers temperature better. That is why scaling up tank size is one of the easiest ways to make a system more forgiving.

Do not run a hydroponic reservoir on faith or a rigid calendar alone. Top up daily with plain pH-adjusted water, take an EC and pH reading twice a week, and do a full change every 7 to 14 days or whenever the numbers drift past your limits or the tank hits half. Two cheap meters turn all of this from guesswork into a two-minute check. To right-size the light driving all that growth and transpiration, try our free light calculator.